I have a flight booked into bkk and out from manila. I'd like the change the return. In the app it wants to charge me a small fortune despite the new return still being in R class. Not helped by the fact they are recalculating the whole flight ( Inc outbound that I don't want changed) If I was to wait until I've flown the outbound is it likely that they use the fares from when I originally bought the ticket?