In reply to wushu:
From acas:
An employer may be able to change the terms of an employee's contract (written or verbal) if one of the following applies:
•there is a clause in the contract that allows them to do so (a variation clause), or
•the employee agrees to the change, or
•the employer gives the correct period of notice.
The correct procedure to use depends on which of these applies.
If none of them applies, then attempting to change the terms could be a breach of contract.
However, if the employer wishes to change the employee's hours or shifts, and the employee is on a zero hours contract, the situation is different.
Note that if the reason for the proposed change is that there has been a transfer of a business (or part of it) from one owner to another, then this may be covered by regulations known as TUPE. If this is the case, it may be wise to take legal advice about whether the change might be unlawful.