Back in 1988 BA had a similar problem when they Merged (Took Over) British Caledonian. They announced to all the B-Cal engineers that their contracts were invalid and they must sign new ones or walk.
Many people signed new contracts and were awarded a golden payment that went some way to compensating them for the loss of a large chunk of their salary that they would suffer on a BA contract.
4th year B-Cal apprentices like myself were told similar stories. Sign and beome a fitter (£20pw more than a B-Cal apprentice rate) or become a BA 3rd yr apprentice on £100pw less.
Following the take over many staff were moved between LGW and LHR without their thoughts and hapiness being taken into account. This was why BA became known as Bad Attitude.
For the following two years (until I could take no more) it was impossible for anyone that was ex B-Cal to get any training or recognition for what they had done.
On the good side, the staff that did not sign new contracts and were told to walk later took BA to court. Having spent 18 months or so contracting around the world, BA were ordered to take them back in their original posts at their original salary with back pay.
Bad Attitude seems to have some back!