Change Seat on BA flight after AA flight (within 24 hrs of flight)
This is somewhat finicky. You need to allow time for the AA check-in status to update the BA booking, and that can take a few minutes or even hours. Then using just the BA PNR, use the BA App to hopefully select seats. It doesn't always work, eg if your Advance Passenger Information isn't correct. You can not ring to fix this either.