Actual deleting would probably need talking to one of the head mods.
From what I hear, though, that requires a very good reason and you have to wait quite a bit for response.
Best bet is to simply go edit your posts to 5 characters (like ***** or ----- or something) and forget it existed. If anyone did a reply with a quote of what you wrote, however, that will always be around.
If its just to swap names, don't worry about it. Go change your signature on the old name to "I'm now _____; talk to that one."
I had to do that when my old name started having strange issues.
