Well I took the flight, and I heard the guy in "my" original seat tell a colleague he had nearly a million airmiles (presumably on BA), so he must be at least Gold card holder and maybe they can do that. He also said he had hoped to get upgraded, though obviously he didn't.
The seat I got instead was fine in itself but that won't always be the case and the possibility of being d1cked around like that makes having a Silver card less valuable, so I may be going elsewhere for my long haul flights in future.