This thread is full of valuable advice, and you should probably bookmark it and read all of it again at times. But if I had to pick a single advice as the most

This thread is full of valuable advice, and you should probably bookmark it and read all of it again at times. But if I had to pick a single advice as the most important one: lose the term "rewrite". It's way to easy to get a deadline or time budget attached, and that's a very bad place to be in, especially if people start to think the rewrite stalls regular development. Improving an existing codebase is not a task, it's a process. You make it part of your everyday routine, deliver features on the way, and your budget for improvement is virtually unconstrained.