I've just changed my username from "Rumbler" to "Bumbler" to keep "rambler" happy, and my username changed automatically not just on the thread we were on, but back to every post I've ever made...even weeks ago. Weird.!
not quite. if you have been posting as Peter and edit your name to Fred say; the moment you change your name to fred ALL the previous postings in the name of Peter are automatically changed to Fred. if you re-edited to Peter all the freds will change back to peters.
you can have as many registered names as you can be bothered to create iffy e.mail accounts for. keep some spares and you'll always be able to come in with a new name not used before.