A downgauge from a 77W to 772 is not uncommon although it's probably less driven by short-term overcapacity and more just regular operational considerations. And remember, the planes "start" in LHR and don't actually spend too much time in JFK before flying back to LHR. There are people flying in both directions (duh), so a light load on one flight is literally only half the story. That said, when they do downgauge or if there is a cancellation, it's not usually an issue to absorb passengers on one of the other flights. They run every 30-45 minutes between ~6pm and 11pm, including the AA metal flights. So you likely won't even get any EC261 compensation from a cancellation.