Link your xbox live account to your passport account now so you can get to it easily from the 360, then delete the account record from your xbox.
I don't mean to be pesty, but....
bump
I've looked all through xbox.com, but I just can't seem to find it.
Ok. Found it for you.
Go to xbox.com and then click on the top right button which reads 'Sign In'. You then need to log onto your passport acocunt (if you dont have one create one) and then you'll go through various steps - one of which is Linking your Gamertag.
Adam
I imagine you are now linked. However, I do not know if that is all.
I would try contacting M$ and explain your situation and what you have done so far - just to confirm.
Adam