AI is just one tech, there's been "language X to Y" converters for a long time, including Cobol to Java. To the point where it will compile and at least seem to do the same thing, but... that's the thing with these codebases, verifying that it does the same is the challenge.

I have 0 experience in this field, but I'm willing to take a guess that the majority of a Cobol to X developer's work is not (re)writing code, but figuring out what the original code does, what it's supposed to do, and verify that the new code does the same thing. More testing than programming.