Instructions
Turn on your Xbox 360 and wait for the dashboard to load up.
Press the Xbox logo on the center of your controller to open up the "Xbox Guide."
Select "Recover Gamertag" on the guide. Select "Recover Gamertag" again to begin the process.
Select "Yes" to end the current session. Enter the email address and password of the account you now have registered to the Xbox 360 gamertag.
Wait while the gamertag is recovered on to your console.