XBOX Live's Windows Live ID change

Have you guys ever been able to change your Windows Live ID that is linked to your XBL account? I don't use the email that I used for it anymore and I want to change it. I went to the dashboard and it does have that option but it kept saying the new email I was trying to enter was "invalid." WTF?

Same thing happened to me a while back and I called XBOX Support (1800-4MYXBOX) and after a (very very long) talk-through, they were able to help me out.
I dont remember what they did, all I know is they did it, so that is always an option if you can't figure it out.