Employers can only amend the terms of an employment contract if they have reserved the right to change them in the contract, and only if they do not act in an unreasonable manner.
If they have not reserved the right to amend the terms of the contract, then they cannot do so without the employee’s consent. However, the outcome will depend on the extent of the amended terms. For example, a pay cut would typically result in an employee resigning, whereas an increase in paid holiday entitlement would often not justify an employee resigning because it benefits the employee.
It is best to discuss any changes to the employment contract with the employee and explain any reasons for the amendments.