Its not possible to change User Name. At least it is beyond the power of a Moderator to do so.
You can ask to have original user name banned and then re register new name. If you simply try to register new user you run the risk of both users being banned.
I'm sorry but the technical issues you hav experience are beyond me. Hopefully someone else might be able to assist.
“Life is a trick, and you get one chance to learn it.”
― Terry Pratchett, Nation