Thanks for your patience. Changing his working days would amount to a change of contract so it is not something you can just go ahead and do. Ideally you want to obtain his consent on doing so but as you can imagine he may not be too happy about it and could refuse.
If they refuse you have to consider the best way of trying to enforce these changes. The most legally sound way would be to issue them with notice to terminate their current contract and then once that notice expires - re-issue them with a new contract which includes the new hours. You therefore have to see what contractual notice they are entitled to (it must be at least 1 week for every full year of service) and then issue that. You must continue paying them and allowing them to work under the current contract’s terms until the notice ends.
The risk with that is they would potentially be able to claim unfair dismissal, but if you have a sold justification as to why you needed these changes, such as business needs which there was no way round, then you could possibly defend it.
Once the current contract has ended and you issue them with the new one you could vary their hours of work and if needed reduce their pay accordingly. However, if you decide to only reduce the hours and keep their pay the potential of a claim will be significantly reduced as they will not be losing out financially as a result.
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