Unable to change back the way of capital letter(s) in username.

I've recently changed my username to 'all caps' and i after a month and half, I've decided to change back to the generic version of my username which only has the first letter of it  being a capital letter but for some reason, I'm unable to change it back and would just appear with a message "This profile name is not available", I hope this bug would be resolved soon since i really want my old name back, that's all, THANKS!