I don't have much knowlege of corporate takeovers / mergers, but I would have thought that any recruitment / expenditure / assset transfers / t's and c's changes etc etc, which are conducted in the perioud between the agreement to marge / takover and the actual merge would be reviewable and reversable. otherwise it would be very easy for a company to agree to a sale / takeover and then ofload / onload a whole bunch of stuff and screw the new owners. I know BMI have an operation to run in the mean time, but I'd be very very surprised if IAG and BA haven't reserved the right to change / reverse anything that happened between the agreement being signed and the actual merger taking place.
good luck to the guys that were already in bmi and are joining BA, but people joining now smacks of a bit of a dodgy 'back door deal' into BA.