If whoever has the later flight is prepared to spend a few extra hours in the lounge, just ask at check in. The rare answer would be to make this change with fare difference or fee, but there are occasions when it works in BA's favor too and the later passenger will find himself on the earlier flight.

For the benefit of others, AA will provide SDC on the LHR-JFK service for $150 with that fee waived for higher level elites and their OW counterparts.