I doubt LLMs are being used much for COBOL, if anything it’ll be used in a “replace this COBOL with Java”. COBOL is used as legacy code and most of it has already been rewritten in Java.

As someone who knows COBOL, I’d say, don’t bother. People over exaggerate the availability and remuneration of COBOL jobs. Maybe they used to be well paid or there used to be lots of jobs, but it’s pretty much a myth these days.