BA apparently want this new contract to save them money simply through volunteers. In order to achieve this, they only need to pay the same money per crew member, but gain efficiencies so that they no longer need so many crew. This can be achieved by having the mixed fleet with different contact terms.
They will probably save even more money, if they slightly up the new contract overall pay so that they get even more volunteers, and hence more efficiency. If they get enough volunteers, the pay for the old contractors will go down considerably, because they will be given as little lucrative work as possible. With sufficient volunteers, BA probably do not even need the old contractors to agree to a new contract, because their allowances will be reduced dramatically.
In my opinion BA are being very clever with all of this. There is no way that confidential Columbus literature was leaked to BASSA by accident. They are just gauging the reaction, and will more than likely offer a final package that is actually quite attractive to the majority of BASSA's members. They want it to be voluntary. Therefore by definition, it must be more attractive to at least some crew.