I have a couple of questions about seat selection for an Avios booking in Club World (LHR-IAD/DCA and LAX-LHR).

I understand that I can pay to select a seat at the time of booking, just as if I was booking with cash (can you confirm?). However I understand from searching the forums that much of the Upper Deck (for 747s) will be blocked off, as I don't have status with BA. If I book nice and early, what is the likelihood of getting a decent seat upstairs (ideally the backwards-facing window seats by the emergency exit, or more realistically an aisle seat)?

And if I pay to select a seat, can I then change it during OLCI if a better seat suddenly become available?

Thanks.