I'm with Cingular and just changed my username for my BIS account. Cingular is the only one that can delete the old account.
There are two ways to do this. Call in (and wait). Or log onto the online tech support chat for devices (very fast).
I used the online tech support chat.
They deleted it at my request and I simply started fresh and setup a new BIS account: Total Time: 8 minutes.