Instructions
Connect to Xbox LIVE and sign in using your current Xbox 360 Gamertag. You must be connected to the Internet to connect to Xbox LIVE.
Scroll down on the home screen to the "My Xbox" tab. Select your Gamertag from the submenu on the tab.
Highlight the option "Change Gamertag" and press the "A" button to confirm your choice.
Select "Enter New Gamertag" and change your Gamertag to what you want to use with your "Final Fantasy XI" account. If this is the first time you have changed your Gamertag, it will be free. However, if you have made the change before it will cost you 800 Microsoft Points to change it again. if you do not have the points click "Add Microsoft Points" and purchase them for a cost of ten dollars, as of publication.
Confirm the changes to your Gamertag by selecting the, "Yes, Use This Gamertag" option.
Play "Final Fantasy XI" on Xbox LIVE to view your new Gamertag.