I don’t remember which books exactly, but it was about legacy code. The author was talking about seams. If you’re trying to complete a refactor in one go, it will usually be arduous and maybe fail. What you want is to find seams in the codebase and maybe create some, where you can gradually decouple the architecture. After that you’ll have contracts and independent modules that can be reworked.

It’s not easy though and you need to have a very good understanding of the codebase. But the nice thing is that you can do it without having to maintain a long lived branch (which is an antipattern).