When I told them about it, their response was pretty much "Oh, too bad - once that username is created, it is there for life. You will have to shut down that account and create a new one", meaning I lose all the benefits I got from the promotional transfer.
So I just put up with a wrong first name whenever I sign on or ask for support from them. I still think it is strange that some second or third level support engineer can't just change that info in the database for me, after the proper authentication. I can't believe that something so easy to create over the phone can be so hard to change over the phone.