Can you make sure the password fields are empty when you attempt to change the username?
I have tried this, it tells me to enter my current password, when I do, it tells me its incorrect, however I know for a fact it is. I have even copy/pasted to make sure i wasnt typing it correctly, I have also tried it on 2 different computers and 2 different internet connections, same thing on both.
Edit: I got a response from my ticket, but its the most unhelpful canned response Ive ever gotten, and is not even remotely related to my issue.