Hi,
The answer here might be disappointing, but the employer does have a right to change contract and I am sure they know how to go about, in a correct and legal way. You should both agree on the new terms. If you don't agree but continues to work there, it will be seen as you do agree after all.
If your employer changes, you are normally entitled to receive a new full written statement of employment particulars within two months of the change. You would not be entitled to this if:
your employer’s name changes without any change to who your employer is (for example if the company is renamed)
your employer changes, but your job continues unaffected (such as during protected business transfers, also known as TUPE)
In these circumstances, you are entitled to be given individual written notice of the change at your employer’s earliest opportunity. This must be no later than one month from the date of the change.
Hope this helps a bit!