Changing employees’ terms and conditions is unlawful in common law if you do not have the agreement of the employees involved to make that change. This would be a unilateral change and would constitute a breach of contract.
If employees do not appear willing upon first mention of the change to, in this instance, their working hours, then you should enter a period of consultation with them. Your consultation should be meaningful ie you should enter into it with an open and take into consideration anything the employee may say which means that the change does not need to be made.
The reasons for the change need to be good business reasons ie there are external reasons or demand which is not currently met by the current working hours and this is having a detriment on the organisation. Having this kind of justification may help your position should the situation go further.
Providing notice of the change is always advisable but there is no prescribed minimum. Offering an incentive may make the change a more attractive proposition but there may still remain some staff who will not provide their agreement.
Ultimately, if you don’t get agreement, you could serve notice to the employees on their current contracts of employment and then re-engage them on contracts under the new terms. This is still a dismissal in law but, provided your reasons are good enough, a tribunal would find a fair dismissal.
Ultimately it is for the employee to take some action in relation to the change but there are time limits involved.