As far as I know its not possible. The easiest thing to do would be just to start a new one (new log-on etc) and ask Andrew (vanderaj) to transfer your record of post numbers.
The user name is not the primary key on the members table so it shouldn't care if I chnge it. That being said I hve never changed one but it shouldn't be a problem.
What would you like it chnaged to ??