Hi pandaperth, yes you are correct, it will be the net of the difference in TFCs, plus the change fee. BA tends to collect the change fee with one hand and refund any residual due with the other, meaning if your net TFCs are a refund of USD120, your end result will be an extra USD5 on your credit card but via two transactions; a refund of USD120 and a charge of USD125. End result is the same of course.
Yes, BA do collect the YQ/YR for RJ/QR. I just priced some segments in Amadeus to test.