The problem you are running into, is one of where your current ID is showing up as you already being resgistered so the new application is being rejected. This is done to prevent shade accounts on the bulletin board.
What we had tried to do was to just close the old account to allow the new account to be used. However, that created another problem. People have thier browsers set to automatically logged them into the bulletin board cannot get back in once the change as made, as they have no idea how to clear a cookie.
Too many people want to change thier bulletin board ID's a week, so it is too much of a support load to manually change it for them.
Right now, you cannot change your existing bulletin board ID. I am exploring a way to allow users to change thier ID themselves, but I that is still in research mode.