If you have reserved the right to amend the terms within the contract, the answer is yes, provided you are not acting in an arbitrary or unreasonable manner.
If you have not reserved the right to amend the terms, the answer strictly speaking is no, unless it is with the employee's agreement or consent. The outcome, however, will depend partly on how serious the change is. For example, a cut in pay rates would normally justify an employee resigning and claiming constructive dismissal - and although it would now be necessary for him (or her) to raise a grievance, in a case like this it might well be done after resignation, by the 'modified' procedure (see Discipline and grievance). On the other hand, the recent increase in minimum paid holiday entitlement (to 24 days, for someone working a five-day week - see Holidays) would not justify anyone resigning, although it is certainly a material change in the terms of the contract and can be done without bothering to amend it, because it is to the benefit of those employees affected. (It does require a letter to inform the employees in question of their new entitlement.)
If a change is introduced and the employees do not object, even if there is no formal agreement to the change, they may be taken to have agreed to it by carrying on working, particularly where the change has an immediate impact.
Whatever the circumstances, the best approach is to discuss. Explain why you want to make the change, and offer incentives if necessary.
