If you want to change the Windows Live ID that you used to access features like Xbox LIVE, Marketplace and Zune Pass from your Windows® Phone 7, you’ll need to perform reset your phone.
Other email accounts, including additional Windows Live accounts, and Exchange accounts can be removed without resetting your phone.
WARNING: Resetting your phone will erase all your personal content, including apps that you purchased and downloaded, and will restore the factory settings. You may not be able to transfer purchased content to a different Windows Live ID.
If you are sure you need to change the Windows Live ID for your phone, go to Settings then to About and press “reset your phone”