I haven't seen any user ID change on the forum so far but for username change you can send a message to Theymos from the original account. You should also state the reason to Theymos that why you want to have your username changed and if he get convinced then he may change your username.

I'm pretty sure that Theymos also has the power to change a user ID but I'm not sure he'll do that for you or not as I haven't seen him doing that for anyone else. Or even he has done that for someone then I'm not aware of it. But, surely you can send a private message to him and ask for the username change. You won't be disappointed if the reason for changing of username is valid.