I have had a 928 for about a month now and have not been able to figure out (through forms at Microsoft or Verizon) how to change your Xbox Live account from one email to the other. My sons login is through my MSN account, and my login is through my Google account. Would be nice if you had the ability to log out on the Xbox Live tile, or somewhere else in the settings options for Xbox or email. If anyone has a solution please let me know.
If it's anything like an Android phone, the address is used to sync the information from the phone, including any apps you may purchase. Most likely, you would have to reset the data on your phone and start over with the new email address. Unfortunately, this will mean you will lose everything on your phone.