Let’s say I change my user name to newusername . And then someone does a search for all posts by newusername . Would the posts by plan b come up?
I believe so.
For example, Master Wang-ka used to be Wang-ka, but I’ve been able to search under his new name and come up with his old posts.
Yes. I decided a few months ago that my old user name was a bit too like my real name, so I asked to be renamed. All my old messages automagically had my new name attached.
Remember that the only way to change your user name is to email an admin. And… you only get one change, so make it good.
All your previous posts will come up under newusername.
In threads where someone has referred to a previous post by quoting or simply typing your (old) user-name, your (old) user-name is part of the text of the message, and thus subject to text searches.
This can also be confusing in threads that refer to past posts extensively.
So if I reference the post by ivylass, above, her name is now part of this thread text. If she were to change her user-name, say to just ivy, the member hyper-link would say ivy, but this post would still say ivylass. Twice.
Actually, we’re a little more flexible these days . . . if you’ve had a name change before, we’d consider changing your name again.
We’re just not comfortable with people changing their screen names like they change their underwear. Names have meaning and we want you to have a name here you’re happy with and that people can identify you by. However, we also understand that people do change, so we’re not necessarily confining you to one change in your life here at the board.
But not once a week, neither. 
your humble TubaDiva
Cyborg Platypus is still available.
Or changing underwear like screennames 
Well, the ‘II’ surrounding my changed nick, were forced by the fact that the user Gyan exists. But he has never posted. Ever. If it’s ok, I’d like to adopt that nick.
Which moderator should one e-mail in order to change their username? I also made the mistake of using my real name.
Any administrator will do it for ya.
Yeah, the name I wanted was registered in 2000. He posted three times in 17 hours in the same thread, and has never been seen since.