You need to request a new username via the Help Desk so it will match your current Xbox Live Gamertag.
Here's a direct link to the forum:
http://www.xiledgaming.com/forumdisplay.php?5133-Request-Login-Name-Change
Here's a direct link to the forum:
http://www.xiledgaming.com/forumdisplay.php?5133-Request-Login-Name-Change