Originally Posted by flarmip

The best option here, in my view, would be to either rebook the whole trip through BA, if you have enough Avios and they are showing availability for the desired AA sectors.

As a second best option, buy an Economy Plus or Business class cash fare through BA's website for LHR to EDI. Choose a flight that gives you a very comfortable connection off your JFK arrival, say 4-5 hours later, in case of any delays. Remember that since you're on separate bookings, if you have checked bags you will need to collect them at T3, take the train to T5 and then check it in again with BA there. Regardless of whether you have bags (in which case you need to go landside) or not (in which case you can transfer airside via the bus) you will go through passport control and security.

With an Econ Plus or Biz cash fare you get what's known as FCOD (Free Change on the Day), which allows you to make free confirmed changes to another flight from midnight UK time on the day of departure. So as long as other flights have availability on the day, you will be able to change for free to an earlier flight. You can change multiple times, but each change must be done at least 1 hour before the scheduled departure time of both your current flight and the new one you want to book onto.

Online check-in closes 1 hour before departure and the check-in desks at LHR close 45 mins before departure for short haul flights such as this.