Comment by zurfer

I heard the story once on how you migrate these old systems: 1 you get a super extensive test suite of input - output pairs

2 you do a "line by line" reimplementation in Java (well banks like it).

3 you run the test suite and track your progress

4 when you get to 100 percent, you send the same traffic to both systems and shadow run the new implementation. Depending on how that goes you either give up, go back to work implementation or finally switch to the new system

This obviously is super expensive and slow to minimize any sort of risks for systems that usually handle billions or trillions of dollars.