Originally Posted by
northeastflyer
Yes I could do that, but I'm not going to finalise the change until I know they're confirmed, so that's why I'm asking if the BA agent can book the flight segments then reserve the seats before we finalise the changes.
I don't think it practical terms it can be done. You can hold a reservation without seats and without payment, but to hold seats you need to have a booking, and to have a booking BA would like some form of payment. What you are asking for (essentially only switching if a particular seat is available) is sufficiently obscure that the logic wasn't built for it, given it's easy enough to see if the seat is actually unsold. If you are covered by BWC then you can always switch again.