Let's say I book LUX-LHR-JFK-SFO for Summer 2021. LUX-LHR-JFK on BA, JFK-SFO on AA. I'm going to assume there will be some form of schedule change on at least one of those flights over the next ~9 months. Assuming this is true - would BA (or AA) allow me to change dates of the JFK-SFO (or SFO-JFK) segments so that they become stopovers rather than connections (e.g. I book LUX-LHR-JFK-SFO for June 1st and returning June 30th but then there's a change JFK-SFO, would I be allowed to keep LUX-LHR-JFK on June 1st but move SFO-JFK to e.g. June 20th)?