If I remember correctly the underscore is in fact the problem.
As far as I know you can not change the login-name yourself (just the shown username), it can be done by an admin, but would reset your passord and generate a new one, which will be sent to your mail-address, so it must be valid.
So changing the name might not work without the "help" of an admin.
If you want me to do that, please post it here or sent me a pm with the new name.