If you’re responsible for legacy IBM mainframe systems, you’ve likely faced an impossible choice: continue paying escalating maintenance costs for aging infrastructure, or embark on a multi-year, multi-million dollar modernization project with substantial risk of failure.
You’re not alone.
Industry data shows that many organizations that have attempted IBM mainframe modernization once, or several times, have had to abandon the effort after months or years of work with little to show for it.
So what ails mainframe modernization?
Legacy mainframe systems weren’t built with extensibility, documentation, or future-proofing in mind. They contain thousands, or even millions, of lines of tightly coupled code with minimal annotations and little knowledge transfer from the original developers who probably retired decades ago. Documentation hasn’t kept pace with decades of modifications either.
And if you were to reverse-engineer these systems manually, it would take an army of specialists and years of painstaking work. The talent shortage is real. As the developers who built these systems are retiring, finding engineers who can read COBOL, Natural, JCL, or other mainframe languages is becoming more difficult every month.
That’s not all. Mainframe hardware requires periodic upgrades that come with substantial licensing costs, often forcing decisions within 1-2 year windows.
As a result, many organizations prefer to leave their mainframe system running as it works today and decide to throw licensing and hardware money at it instead of risking breaking things.
The risk of modernization has historically been even more daunting than the cost of standing still. When a state agency’s payroll system processes payments for 2 million people, or financial systems handle billions in transactions daily, the tolerance for failure is zero.
This reality has kept critical business systems trapped on mainframes well past their intended lifespan — an approach that’s unsustainable as these systems are not built for AI, or modern cloud services, APIs, or data analytics platforms that drive competitive advantage.
Ironically, Generative AI, the very technology it can’t support, can fundamentally change this equation making modernization projects that were once economically unfeasible now practical and achievable.
Mainframe Modernization Looks Different Today
The traditional approach relied on large teams of expensive mainframe specialists spending months in reverse-engineering phases. With AI-assisted development, the time it takes to complete these phases has shrunk dramatically while requiring smaller, more cost-effective teams (from our experience).
At OZ Digital, we have a proven methodology that uses AI tools, like GitHub Copilot and custom AI agents, to automate the work—that once required an army of consultants— without sacrificing quality and helping reduce risk. Once the AI tooling is in place, the specific AI model (LLM) used can be refined and updated over time to take advantage of newer model capabilities.
AI-Assisted Mainframe Modernization
A key to our modernization methodology lies in the integration of AI with a “human in the loop” where their experience and intelligence is augmented. We’ve seen strong results with this approach where the capabilities of human developers result in increased speed and accuracy. We’ve developed a proven methodology where legacy modernization has distinct phases, and AI serves as a force multiplier every step of the way.
1. Bridging the knowledge gap
Many of your developers may have limited or even zero previous experience with mainframe systems (COBOL, Natural, etc.). In traditional modernization projects, this would be disqualifying. You would be required to hire expensive mainframe specialists or invest in months of training for each developer.
Instead, you can use AI as the mainframe subject matter expert (SME) who understands legacy systems. The AI can be used to perform:
• Extraction and documentation of the mainframe codebase
• Answer questions about business logic and system behavior that’s embedded within legacy source code
• Clarify communications with the mainframe team
Meanwhile, GitHub Copilot and custom AI agents help modern developers understand the legacy code without becoming mainframe experts themselves. The AI explains what archaic code patterns mean, identifies business logic buried in procedural code, and highlights dependencies that weren’t obvious from static analysis.
2. Automated code conversion with human validation
GitHub Copilot custom agents enables us to create specialized AI developers that perform entire conversion workflows autonomously. These agents:
• Analyze mainframe (COBOL, Natural, etc) source code to understand program structure and logic flow
• Convert procedural mainframe code to object-oriented modern code; like C# and ASP.NET
• Generate modern web interfaces to replace terminal-based screens
• Write unit tests to help validate the business logic is implemented in the modern code
• Map mainframe database (DB2, Adabas, etc) database schemas to modern databases; like Azure SQL Managed Instance or Azure Cosmos DB
A critical element to this entire process is maintaining the “human in the loop” oversight. After AI converts legacy, mainframe code to modern C# or Java code, human developers review the converted code for accuracy and idiomatic patterns, test functionality against the original system behavior, manually test and validate using real data from the database, ensuring that all business logic has been captured correctly. Additional unit tests are written with the help of AI to verify business logic implementation and edge cases.
This hybrid approach gives you the speed of automation with the reliability of expert validation. The AI handles the tedious analysis and translation work that would take a large team months to complete manually, while human developers ensure nothing critical gets lost in translation.
3. Understanding Complex Legacy Logic
Mainframe code, like COBOL or Natural, looks fundamentally different from modern languages like C#. Control flow, data structures, and programming paradigms that were common in decades past bear little resemblance to contemporary software engineering practices.
AI tools prove instrumental in helping dev teams understand the business logic and execution flow of legacy code. Rather than spending weeks mapping dependencies manually or creating documentation from scratch, AI leverages five key capabilities:
Code explanation and contextual summarization: Developers could point GitHub Copilot at complex mainframe code routines and get line-by-line explanations of what’s performed. This dramatically reduces reliance on out-of-date written documentation and allowed modern developers to contribute meaningfully to code understanding tasks.
Automated documentation generation: Instead of manual documentation that quickly becomes outdated, GitHub Copilot can generate documentation by interpreting the intent behind business logic within the legacy code. This enabled the team to build updated documentation for use in project handoff and training.
Pattern recognition for modularization: GitHub Copilot and AI agents identify repeated patterns and redundant logic across the codebase, helping to understand where monolithic mainframe programs could be decomposed into smaller, more maintainable modern components.
Dependency mapping: Combined with static analysis tools, AI enhances the understanding of legacy code interdependencies, call hierarchies, and data flow. This makes it easier to isolate functionality for migration.
Data flow analysis: Traces how data moves through the system from input to output, understanding how business rules are applied at each stage and how different modules depend on shared state.
The analysis phase traditionally consumes 50-75% of a modernization project’s timeline. Through AI augmentation, we’ve seen reductions in this phase from months to weeks—a direct result of AI augmentation reducing time-consuming and tedious manual reverse-engineering work.
4. Risk mitigation through comprehensive testing
Rigorous testing and user acceptance testing (UAT) are critical to validate business logic and get custom business sign-off before the final production release. Depending on system criticality and downtime tolerance, organizations can choose phased migration or full cutover approaches. Regardless of the approach, there are several things required during this time:
• AI-generated test cases covering expected behavior from the legacy system
• Automated regression testing comparing outputs between legacy and modernized systems
• Extensive UAT with end users validating business processes
• Data migration validation ensuring complete and accurate transfer from legacy mainframe databases to modern databases
The final cutover involves migrating production data and switching entirely to the new system. This can be a high-stakes transition, but AI makes it feasible by the confidence built through thorough AI-assisted testing and validation.
5. Delivering business value beyond code
Technical modernization is the means, not the end. The business outcomes justify the effort, while they extend far beyond simply replacing one technology with another.
6. Seeing immediate financial benefits
The most immediate outcome executives care about is a clear reduction in ongoing mainframe costs and the long-term financial risk that comes with them.
Within 1-2 years, the business will likely face mandatory mainframe hardware upgrades and associated licensing costs. The modernization project eliminates these recurring expenses entirely. For organizations running large mainframe footprints, these costs often reach millions of dollars annually.
By migrating to cloud services, like Microsoft Azure, the business will replace high fixed costs with variable costs that scale with actual usage. For a mainframe system with predictable workload patterns, this represents significant long-term savings.
7. Unlocking operational advantages
Beyond cost savings, modernization unlocks day-to-day operational improvements that make the system easier to run, scale, and evolve.
Using Infrastructure as Code (IaC) tool like HashiCorp Terraform, makes infrastructure changes auditable, repeatable, and version-controlled. This eliminates the “Snowflake Server” problem where production environments drift from documentation. IaC also makes it much easier to deploy multiple environments (Dev, Test, UAT, and Prod) that are all configured the same.
• Modern DevOps Practices: The new system integrates with contemporary CI/CD pipelines, automated testing, and deployment automation. What once required manual coordination and weeks of planning now happens through automated workflows.
• Improved System Reliability: The built-in redundancy, automatic failover, and disaster recovery capabilities of the cloud exceed what most organizations can achieve with on-premise mainframes. The business gains SLAs and recovery capabilities that would have required substantial additional investment in traditional infrastructure.
• Better Monitoring and Observability: Modern APM (Application Performance Monitoring) and logging tools provide visibility into system behavior that legacy mainframes rarely achieve. The operations team can now identify performance issues, errors, and anomalies automatically rather than waiting for user complaints.
8. Creating strategic capabilities
Beyond operational stability, modernization creates new strategic options that expand what the business can deliver and how fast it can move.
API-First Architecture: The modernized system now exposes APIs that integrate with other systems. Legacy mainframes are notoriously difficult to integrate. This modernization opens new possibilities for automation and interoperability.
Cloud Scalability: The system can dynamically scale compute resources during peak workload periods and scale down during quieter times. Mainframes require fixed hardware provisioning for peak capacity, leaving expensive resources idle most of the time.
Foundation for Innovation: With modern data access patterns, the new system enables advanced analytics, reporting, and integration with emerging technologies like AI-powered auditing or fraud detection. The old system would have required years of exploratory projects to achieve similar capabilities. It will also be possible to integrate a data lake for expanded analytics capabilities.
Talent Retention: Modern developers prefer working with contemporary technology stacks. By modernizing the system, the organization makes existing IT roles more attractive and eases recruitment of skilled engineers. This is a strategic advantage often overlooked in cost-benefit analyses.
Why choose the OZ methodology for mainframe modernization?
Our methodology is more than just another consulting approach. It presents a fundamentally different way to modernize legacy systems.
Here’s what our methodology outlined here brings to your project:
AI-first methodology:
Unlike traditional consulting, we automate the analysis and conversion phases using GitHub Copilot and other custom AI agents. This has greatly reduced project duration and costs compared to conventional approaches, based on our experience.
Smaller, more efficient teams:
Utilizing AI tools will enable smaller teams to be more productive as compared to a traditional, manual modernization. AI helps reduce labor costs while maintaining Smaller, More Efficient Teams: quality through AI-automation and rigorous human oversight.
End-to-end modernization:
From comprehensive assessment through cloud architecture design (Microsoft Azure, AWS, etc), data migration, and knowledge transfer.
Predictable outcomes:
Our experience and proven methodology delivers consistent results. We have experience working to modernize large 2M+ lines-of-code legacy codebases, comprehensive testing, and zero-downtime deployments.
Risk mitigation built In:
Rigorous AI-assisted testing and human validation ensure business logic preservation without the failure risk of previous modernization attempts.
Whether you’re running COBOL, Natural, or other legacy languages on mainframes, this AI modernization methodology will help you achieve more with less, and finally migrate away from that aging mainframe.
Take the First Step
If you’re facing escalating mainframe costs, struggling to maintain legacy systems, or planning for inevitable hardware end-of-life, now is the time to explore modernization options.
As a long-standing Microsoft partner, we help organizations modernize their environments with clarity and intent while balancing innovation with security, governance, and operational continuity.
The organizations that successfully modernize their mainframes and remain competitive won’t be the ones with the biggest budgets or the largest teams. It will be the ones who leverage AI with “human in the loop” to multiply their teams’ capabilities.
