A couple of times I've phoned to UUA or change a reward booking and the agents involved both mentioned to me that they would "quickly grab the rewards seats before making the booking" or words to that effect. It sounded to me like they were doing this before making any changes whatsoever to my existing booking.

My assumption was there is some kind of mechanism for BA booking agents to remove reward seats from being available/visible to both the public and other staff members so they can make a redemption booking or make changes to an existing one without the worry of the seats being taken by someone else by the time the make the final clicks to confirm everything and take payment.

The last time I did this was four years ago so things may have changed since then.