Switching carriers fuel-surcharges will probably be different but please report how BA change it in practice...If I were to guess, they would add your flights and come back with price change in several days. If you don't like the price, I guess you can keep the original booking. I had a test date change (same flight #), and they confirmed there'd be no charge, but came back 2 weeks later with 150$pp charge for whatever reason. After pushing back they relented to 25$ phone fee which is waived for Golds, so it was 0$ in the end, confirmation date changes for same flights are free and the re-issuance of tickets. However, there was a lot of back-forth between their internal departments.