What you can do depends upon the exact terms of your contracts. If the contracts allow for this variation then you are not talking about changing contractual hours.
Do you have a minimum number of hours set out within the contract? If the contract doesn’t guarantee the hours and all staff are aware that these hours will fluctuate in accordance with seasonal demand then there is no real reason for them to object.
Discuss the situation with your employees and see if you can come up with a workable solution and explain that if you are unable to make this change then what you are looking at is potential redundancies. If they won’t accept the change then this could ultimately result in employment terminating. If an employee challenges this you may end up having to demonstrate to a tribunal that your actions were reasonable and in keeping with the contract.