Instructions
Download EzGt 2.2 to your computer.
Connect your Xbox hard drive to your computer via the transfer cable and open Xport 360. Double-click "Partition 3," then "Content" and then open your profile. Double-click the "F" folder, then the "0001000" folder. Copy your profile file to the desktop.
Open EzGt 2.2 on your computer and click "Open." Select the profile from your desktop. Use the inbuilt editor to change your gamertag. Click "Save" when you are done and choose a location on your desktop.
Copy the new profile file and switch back to Xport 360. Double-click "Partition 3" and then "Content." Press "Ctrl"+"V" to paste the file straight to the content folder.
Disconnect your Xbox hard drive and reattach it to your Xbox. Sign in and select your new profile.
Open "Call of Duty: Modern Warfare 2," then select "Multiplayer." Select "Xbox Live." Hold the middle button until the menu appears, then sign in with your original profile. The new modded gamertag will appear.