Is your current passport registered on aa.com? If it is, it will make purchase on aa.com easier. If the fare is listed as fully refundable (read rules for it carefully), you will get a (full) refund in the form of payment.
[Since you have a US pp, you will not need a visa for travel to LHR (these have the most intl per day), this is probably your best option.]
Yes, it is, and I'm Global Entry and TSA pre also ( which is another thing I will need to get moved -- but that's apparently easier than my visas).
I am thinking simpler, just booking ticket JFK-YYZ, which wouldn't be codeshared, but would be an outsourced RJ. just a lot of variables.
I guess if I buy fully refundable, then I can rest easy.