But there is one way it can be done - lots of painstaking analysis of the Cobol source code to attempt to extract the business logic (which is incredibly slow).

But there is one way it can be done - lots of painstaking analysis of the Cobol source code to attempt to extract the business logic (which is incredibly slow). At least this is how we did it at my old work where we were replacing a mainframe system. In principle though, this is stuff that an LLM could do - assist with extracting some of this business logic and speed this process up (while porting it to a language that is easier to work with). While the LLM doesn't necessarily understand the business context, it can do basic analysis which will speed it up massively. It can also generate test cases across the whole codebase.