If you change your employee's duties or terms and conditions, you have varied the contract/written statement that exists between you.
A contract/written statement cannot be changed unless both you and your employee agree to the change. An alteration of a contract/written statement may be sought by, either you or your employee.
If you intend to change any of the terms and conditions that are required by law to be included in your employee's written particulars of employment then he or she must be notified of this in writing no later than one month after the change has been made.
If there is to be a change in the contract/written statement you should satisfy yourself that your employee has accepted the change. Your employee does not have to say that he or she accepts the change, they do not need to sign any document or put anything in writing.
Should your employee be dissatisfied with the change to the contract, but continues to work under the new terms and conditions without making you aware of his or her objections, they could be considered to have accepted the change and it would then become part of the contract/written statement.
If your employee is unhappy with a change it is his or her responsibility to let you know. He or she should tell you about their objection and state the reason for it. It would be wise to have your employee do this in writing. You should keep a copy of the letter in case you need to refer to it in the future.
You must not attempt to force a change of contract/written statement on your employee without his or her agreement. Your employee may take legal action against you for breach of contract.