Abstract : Mainframe modernization represents one of the most complex and consequential challenges in enterprise computing, driven by the need to integrate decades-old systems with contemporary digital architectures while preserving the validated business logic embedded within legacy codebases. Systems developed in languages such as COBOL and PL/1 encode institutional knowledge refined through years of operational use—knowledge that is rarely captured in formal documentation and would be extraordinarily difficult to reconstruct through redevelopment. Reverse engineering gives the ability to surface this logic, providing business logic, process flows, and data dependencies relevant for the organization in a systematic way. This involves using the source code and behavioral information instead of any incomplete system documentation artifacts available at hand? The uncovered business logic can then be wrapped up in independently deployable, REST-based service components with standardized interfaces and API-led architecture principles. JSON removes the format conversion overhead of most of the legacy middleware, and cloud hosting in turn allows for dynamic resource scaling, geographic redundancy, and built-in failover, making satisfying enterprise uptime and availability requirements much easier. DevOps and continuous integration further allow for the rapid and incremental promotion of code changes through the application without requiring time for a system-wide deployment to occur. The cumulative effect is an enterprise architecture vision that measurably improves performance, maintainability, scalability, and resilience without the disruption of the day-to-day operation and loss of institutional knowledge involved in wholesale replacement of systems? A preserve-first transformation framework offers the opportunity to stepwise modernize and extend legacy infrastructures into an agile, service-oriented architecture that is able to continuously innovate and adapt to changing business needs.