Update. Just arrived LHR. I had the distinct pleasure of being op-up'd to Club. Happened after checkin and new BP given to me in the lounge. So alls well that ends well. Flights of course were rammed. So they did need to make room for someone from the back?
Bonus was being sat opposite a delightful crew member who was on the 264 from IAD that went tech 90 mins into its flight the previous evening and aborted to BOS. Thought it was odd to see 4 BA tail fins at the airport as I drove round to return the rental car in the evening! Apparently hydraulics and some other systems failed on the ageing 767. Not the easiest situation for passengers he inferred. He said four other crew from that flight were on my plane, so maybe that's where some of the seat juggling happened.