Thank you for writing Xbox Customer Support!
I do apologize for the inconvenience and we at email support line are willing to help you.
I understand that you want to link your gamertag to a new Passport account.
If your gamertag linked to a Passport Network account, you can link his or her Gamertag to a different Passport Network account by using the following steps:
Warning: If you have migrated your account to Xbox 360, the you cannot change what Passport Network account is linked. There are no exceptions to this because it is a permanent link. These directions will only result in a vague error.
1. Create a new Passport Network account.
2. Link the old Gamertag to the new Passport Network account. This frees up the link from the old Gamertag to the old Passport Network account.
3. Link the new Gamertag to old Passport Network account.
Note: You cannot have a Gamertag linked to multiple Passport Network accounts. Each Gamertag can only be linked to one Passport Network account. The reverse is also true, each Passport Network account can only be linked to one Gamertag.
For further assistance, please don't hesitate to write back or call Xbox Phone Support at 1-800-4MYXBOX (1-800-469-9269) at your earliest convenience. We are open everyday from 9am to 12mn Central Time.
To expedite service, please provide Service Request Number 1025859913 when you call.
For more information about Xbox or Xbox360, please visit http://www.xbox.com
Xbox Customer Care Team