My father is actually in the process of this right now. He was one of the few people in his company who worked on the rules coded into the current software written in the 70's-80's. He's the only one who can tell the programmers why certain things were done (if they were significant, or just optimizations to make the batches run in time on the VAX hardware of the time). He's heading up the project to port the whole thing to a more modern system, but he's planning to retire soon...