Instructions
Connect the USB storage device containing your gamertag data to the Xbox 360's USB port, if it is not connected already.
Power on the Xbox 360.
Select "My Xbox" in the main menu, then select the Xbox Live gamertag you wish to modify.
Click "Change gamertag," then "Enter a new gamertag."
Type in the modified gamertag. The Xbox will suggest alternatives, and if you like one of the provided options, simply select it.
Click "Yes, use this gamertag" to make the changes. If this is not your first time changing your Xbox Live gamertag, 800 Microsoft Points will be deducted from your account to make the change. If you do not have any Microsoft Points, you are asked to enter payment information to credit your account. Once changed, your new modified Xbox Live gamertag will indicate your player in Xbox Live multiplayer games -- including "Modern Warfare 2."