There will be plenty of people willing to swop ont the new contract. Most of Gatwick will be willing to move to LHR onto a wider range of routes and aircraft, and are already used to the mixed long and short haul routine. They'll probably still earn more at LHR under the new hourly rate deal, and will be able to go part time too.
The part time option, as well as the mixed fleet flying, will attract many already at LHR, especially the lower paid new contract types, and the huge number of temporary contract cabin crew members who have completed their contracts will in many cases be ready to step in.
A few Bassa (old contract CSD) hotheads will shout and roar "over my dead body" in their usual manner but will have been sidestepped by BA's tactics and will just have to sit at home on their basic until they're made redundant.
Bout time too.