I made a test site for someone using a analytics ID I made for testing purposes, once finished I deleted the ID from my own account and they installed their own a couple of months later, did not notice any adverse effect to my account. So 99.99% certain once you delete it from your analytics account that is the end of it being associated in any way with your account. They give this warning "Important: Please note that once you've deleted your account, that's it: the account is gone, and we are not be able to retrieve any historical data or reinstate your reports.". So I would assume it just becomes dormant, as once you unlink it you can not get the UA-XXXX-Y back and someone else can not register a UA-XXXX-Y that has already been used to themselves.
