Depends on the size of the pile. If it's huge, then you have to use LLM too, with some decent tooling like Claude Code. Let it analyse the code first, and put a summary for you. Then design the proper solution yourself, and either fix it manually, or if it's too tedious, guide LLM to apply the changes for you. Ofc iteratively and don't let it apply changesets without your approval. I had a similar task recently, did it like that