Will not let you change! If you do not like your user name, you need to create a brand new one and start over.
They also will not allow the combination of activity for multiple user id's that are the same person (wvufan76 and tem316 in my case). I am an example of a guy who bought his sports fan son a Christmas Five Pack and after one season he told me HD was not for him. I liked his user name better then mine, so that is where my two Id's came from.