I can't find the fares that you're booked on, but unless they have unusual rules then I think you should have no trouble with what you're contemplating. You should make sure that the change is made before the departure of the JFK-LHR flight, but that's probably your intention anyway. I think that the upwards-only-fare-revision rule may apply, but as you're booked on an A class DIF fare, I suspect that there would be no trouble finding a valid fare to rebook you onto.