Quick update - after successfully changing my username back with the help of github support, I discovered why I had had issues when @emberquill had not.
It turns out that if a repository has enough git clones in a week (100 or more, to be precise) then it is considered popular enough to have its URL reserved (archived, I think the term is). This means that if any of your repositories have their github urls reserved, you will (like I did) be unable to remane your username back to its original name without contacting Github support because doing so would overwrite and archived url. Admittedly it will overwrite it with a perfect copy of what was there before, but github seems to take a better-safe-than-sorry approach.
I would also like to point out that Github support were extremely fast in responding and then fixing my problem. <3