This question already has an answer here:
It would often be convenient to see how the site is behaving for a user at any lower reputation (and without moderator privileges) without having to make a bunch of sockpuppets (which carry risks of accidentally doing things you shouldn't).
(Other privileges like gold tag privileges should also be able to be turned off.)
Presumably the only additional privilege over a user with the nominated reputation would have would be the ability to go back to normal.
I'd envisage something where you choose the reputation and click to remove any other privileges that could change, and that it would be replaced with a button that would undo that change.
Things like votes on posts would still count as votes by that user when they went back to being themselves, posts would still be posts, comments still comments and so on.