They could sack crew and, before BA begins any additional recruitment, ask them to come back on new contract and fleet. If they say no it's their decision and they can't claim anything from the company as they had been offered to come back.
There's always the possibility of if it comes to CR, that the company could turn round and say to the CR crew "you can keep your job if you join New Fleet". It doesn't make it compulsary to join New Fleet, but the company would give them a "choice".
I think that might come under the heading constructive dismissal.