Board Thread:Support Requests - Getting Technical/@comment-39867458-20190712153945/@comment-34888194-20190712154622

Well, which username are you trying to rename yourself?

Fandom prevents people from renaming into a name that seems to be impersonating other user, so if your intended username changing has exact alphabetical spelling as other users but with cap or lowercase letters difference, that's what prevents you from changing your name.