It is whether you look at new fleet as a new entity. You cannot stop anybody being a member of a union. However, you do not have to voluntarily recognise a particular bargaining group. New fleet will not have a recognised bargaining facility, as it is new, with new terms and conditions. The move to it would be voluntary and there would be no negotiating facility in place. The risk for BA would be if 50%+1 of transferees still wanted BASSA representation and could force the CAC into granting automatic recognition.
This is what I think would be the case, but that is based on my knowledge as a rep, not a legal expert, so I could be completely wrong!