I'm afraid you clearly don't understand how the law works. It's unlawful to discriminate, isn't it? No. Actually it's entirely lawful to discriminate because I don't like you, because you drive a Fiat, or because you have blue eyes. It is only unlawful to discriminate on specific grounds.
Employers can make mistakes, and if they do they can correct those mistakes. But this is really easy, and I'm not going to spend my time arguing a negative. If and when the employer refuses to allow the OP her previous terms, which law will she be quoting to the employment tribunal? It really is that simple. Just tell her which law it is she'll be using. Because I'm Bening honest and not saying that the employer MUST do anything, just what they can do. They can agree with the OP that's it's not fair and set things back. They can not agree and do nothing. What they decide to do is up to them. But if the OP doesn't like what they decide then their recourse is the law. And the law with regard to the protection of terms in a TUPE is what I have already set out - it does not apply to a new job.
And it is absolutely easy to enforce a contract change even now - the employer simply had to say take it or leave it. The OP can then literally take it or allow the employer to terminate their contract, then roll the dice on whether an employment tribunal supports them or the employer - and that's a big risk to take given the odds on employees winning tribunals and the fact that the OP would need to get themselves dismissed!
There is a huge difference between what is fair and what is lawful. This is unfair. There's absolutely no evidence that it's unlawful. And if you want to make this personal, I can't believe that I have to explain that fact to an adult.
Now I'm not going to be arguing with you any more. The OP can take your advice and tell her employer they CAN'T do it, and see what happens. If, however, they do it anyway, I hope you'll be finding her possible loss of a job and the tribunal case. People have a right to know what risks they are taking when they assert something. I have made those risks clear - the employer can refuse to budge, the employer can enforce the change, and they can dismiss the OP leaving her to go to a tribunal and take her chances. I'm not saying they WILL do that, only that they can. You, on the other hand, are saying an absolute - they can't do it. Really, does anyone believe that they can't?