I can't give you a 100% answer, but the best advice I can give is to contact AppleCare directly - do so under the plan of the phone you are buying if you have to - and speak with them about the best way to go about it. Get the person's name and record the time you called so that you have a record of the answer. Also, double check with them when the original purchase date was, in case the seller is mistaken.
If I had to make an educated guess, though, after working for Apple in tech support for years, I would say that what you're describing should be fine. The one thing I have never seen Apple make an exception for is purchasing AppleCare past the deadline. I remember once an ex-employee spending hours at the Genius Bar on the phone, escalated all the way up the chain, and he got nowhere, all because he was one day past the deadline.
Bottom line: if you're within the deadline, you should be fine, but the canceling and transferring of ownership could be tricky, so it's worth hearing it from them to be sure.