Hello, I was wondering how do I replace a rear derailleur. Like what do I need to know. Will any rear derailleur work? Can I put a 3x10 rear derailleur on a bike that is 3x7 and I just won't use the full range? I am asking because I am looking for one on craigslist. And anything else I should know?

Also, since I already need to replace my derailleur is it worth it to spend a little more oney and just upgrade to 3x11 speed? What is involved in doing that? Is it just cassette, chain, and the shifter?