If I have a round trip BA award ticket and want to make a change, my understanding is that I would have to pay the change fee plus any difference in fare (fuel surcharge and taxes) and miles. Should my round trip ticket be from JFK-LHR and then returning CDG-LHR-JFK, if I only make a change to the outbound leg (date or class of service), is the change in fare (including fuel surcharges) calculated only on the outbound leg or on the entire trip?