could introduce subtle bugs, so doing it all at once while it's on the front of everybody's mind with as much comprehensive review and testing of parts or the whole to everybody's satisfaction. if you don't do it all at once, you'd need to repeat the same amount of testing multiple times.
>files should be functionally equivalent before and after
when you say something like this, the road you are on is paved with good intentions.
You can also introduce subtle bugs in your own feature development, and if you change formatting there it's also at the front of your mind.
I think the main argument for doing a big bang rewrite is that you have a defined before/after, otherwise you're stuck into an endless in-between.