Not that Bob is not official looking, but here it is from the "other" site boss:
If you need to have a user name change
please use the contact us link at the bottom of the page. Tell us what your current user name is and make a request for a new one. If your desired name is already in use, you may have to choose again. User names are best kept to a short length (under 12 characters) and shouldn't contain any special characters (like % or & or #). They also have to be G rated and may not be e-mail addresses or web addresses.