Had exactly the same problem with our flights to Miami in November.
64 A & B reserved at time of booking, received the same e-mail as you a month or so ago.
Phoned BA to be told that the flight was now operated by a 777 although the seat map shows 747.
Tried to book seats two nights ago & couldn't, called BA yesterday to be told that both flights in & out were now not accepting seat reservations as they were probably going to be changed.
Frustrating, but in the grand scheme of life not the end of the world.