It is a good practice on forums like this one to not use user names that may identify you.
After the registration, the forum doesn’t let you change the user name. It is one of the protection from spammers.
Recently someone asked me to change it for him but forgot to tell me his desired user name, so I just set the forum that the users can change it. If you were thinking about changing your username please do it in the next 7 days, because later on, I’ll remove the option.