Users only have to click Update Username, not Change Username and then type their username.
When a user registers after changing their username, and someone else had their new username before and had also registered but not updated it on the site, the person who used to have the name will get renamed to their correct name and registration will proceed. This also happens on the validation in the form.
Updated some Mojang API stuff.
Please report all bugs to me, this is a 'beta' release you could say as I haven't really tested it as much.