You don't and can't fully replace each layer, at least not in order - you should expect to have three or four ways of doing things in the codebase, that's just

You don't and can't fully replace each layer, at least not in order - you should expect to have three or four ways of doing things in the codebase, that's just the reality of large old codebases. It still beats having your whole codebase be stuck on the oldest of those three or four ways.