I've used Claude Code to do the same (large refactor). It has worked fairly well but it tends to introduce really subtle changes in behaviour (almost always negative) which are very difficult to identify. Even worse if you use it to fix those issues it can get stuck in a loop of constantly reintroducing issues which are slightly different leading to fixing things over and over again.

Overall I like using it still but I can also see my mental model of the codebase has significantly degraded which means I am no longer as effective in stopping it from doing silly things. That in itself is a serious problem I think.