When you register, next to the username field it says "Sorry this can't be changed".
If you look on the Help and Frequently Asked Questions page it says:
1.5) How can I change my username?
Sorry you can't. Because of previous abuse by people reusing usernames and the lack of audit trail if people keep changing names then we do not allow it
--------------------
The Administrator
______________________
Help & FAQ's -
Terms of use