Mainframe modernisation approaches, compared.

Recompile the COBOL on a different runtime and swap supporting pieces, such as the database. You keep the code but leave the vendor. Behaviour mostly carries over; testing focuses on the platform differences.

Translate COBOL into a modern language such as Java or C#. This is where the long-term value is: the system becomes code your current team can read, change and hire for. The catch is proof: refactored code has to be shown to behave exactly like the original, which is why verification has to be built in from the first change.

Rebuild the system from its requirements. It offers the most freedom and the most risk, because the requirements usually have to be recovered from the code first through business rule extraction.

Large language models now translate and explain COBOL quickly. The model alone is not the safeguard: the reliable pattern combines program analysis to give the model structure, execution-based testing to check the output, and human review before anything ships.

Choose by what you need to change: the platform, the language or the design. Then plan verification from day one, not at cutover. Replay-based migration testing checks each step against recorded legacy behaviour, and a parallel run confirms the whole system before the switch.