Shouldn't the LLM have some tool that gives it AST access, LSP access, and the equiv of sed/grep/awk? It doesn't necessarily need to read every file and do the change "by hand".

I linked this elsewhere but, the agent could have a skill to use https://ast-grep.github.io/ to perform such mechanical code changes

That's correct, though you'll still end up needing more than AST/LSP/etc for the same reason AST/LSP/etc isn't enough for me (the human lol), ie comments/docs/etc.