(subject to the usual disclaimers - I'm not a lawyer and it's a while since I've been a union rep)
Ultimately, employers can try and change contract terms, in theory employees can decline to accept the change, but employers can get difficult (e.g. making pay rises / promotion dependent on accepting new terms) and can (legally) go as far as 'fire and rehire' if employees don't agree to it.
You mention a change of legal employer - is this a recent thing, or some time ago? If you get outsourced / re-tendered / taken in house, you are legally entitled to transfer with your existing pay / terms and conditions and length of continuous service (this is known as TUPE regulations - can go in to more detail if you want) - there isn't a time limit after the transfer that this expires (there is an urban myth that it lasts a fixed amount of time then expires, but that's cobblers.)
Some employers will try and encourage people to sign up to their T+C, in some cases they will offer a lump sum to do this.
It's not uncommon for a workplace to have different people on different T+C, although this can give employers an opportunity to play workers off against each other.
Well established 'custom and practice' can become part of terms and conditions of employment.
Is there union representation here? Getting union involved is likely to be the best course of action.