When we booked BOS > LHR back in November we had the 787. Then they changed our homebound departure time to later in the day (which is great), but changed the London bound aircraft to a A330. Then just now they changed the LHR > BOS aircraft to A330 from 787. That sucks. No Wander Wall for the kids on a 7.5hour daytime flight.
Aircraft changes are annoying but they're a fact of life on most airlines and TBH the A330 is more suited to the BOS route than the 787 which was really designed for the long haul routes.
Incidentally the Wander Wall is being retro-fitted to the A330s so you may be in luck if you get a refurbished one.