The LHR-BWI (&vv) service seems to currently oscillate between a 788 and 789 recently (with an occasional 78X thrown in the mix). Travelling in CW (at least) I seem ro find that if your seat # exists on both types, if there is an aircraft change, your allocated seat remains the same.
Recently my 1A became 10A when the aircraft changed from 788 to 789 about 4 days out. I then changed to 7k, but at T-24 they changed the aircraft back to a 788, my seat remaining the same (doh). Mustn't grumble since I suspect that there were a few pax who may have been upgraded, then downgraded again... .