Windows Phone 7–Change Live Id
The main Windows Live account cannot be change or removed like other accounts on the phone. In the Settings – email & accounts, Google and other accounts can be deleted from the context menu. The main Windows Live account is the first account listed with a Windows icon and only sync is available from the context menu.
In order to change the main Windows Live Id, the one used to access Marketplace and Xbox Live, you need to reset the phone to the factory defaults. This, off course, has its disadvantages as all content on the phone will be deleted.
To reset your phone, tap Settings – about. Tap reset your phone and follow the instructions. After reboot, another Windows Live Id can be used in the setup.