You cannot unilaterally change the terms and conditions of employment for your staff, irrespective of how much notice you give.
If you are looking to reduce hours then it sounds like you are in a redundancy situation because the need for work of a particular kind has diminished.
You need to consult with your staff about the situation and see if you can agree any changes that will resolve your difficulties. This could be a temporary change or a permanent restructure.
If a change is agreed then this simply needs to be recorded in writing and provided to the employees.
