I got the exact same change as you this morning - notifying a seat change which under investigation showed a 777 now operating my JFK-LHR flight instead of a 747 (I too booked in J on a 747 in my favourite seat 64K, since F is a waste of money/Avios on this sector).
Being a GGL/CCR and this being an Avios reservation (therefore free change), I emailed the GGL team this morning to ask them to switch me to a 747 service and had ticket issued less than 40 minutes later. I thought this was very fast service indeed ^
For cash tickets, you basically have no allowance to make changes. I would always advise booking North America routes with AA where at all possible, since their T&Cs allow for re-routing/refund if the aircraft type has changed from the one originally booked.