Im not on the Airbus. I used to be. Got quite a few hours in it. Its stated in my licence I have an A320 rating. I believe BA would have great difficulty in saying I did not have the skills to fly an A319/320/321.
I will be signing up for the new part time. Do I want to? Not really, but I will take a hit and save my colleagues. I would imagine the vast majority of pilots in BA will be applying to do the same.