A change to the statement will still require the employee's agreement.
If the change results from the employee being required to work outside the UK for more than a month, and they will be leaving within a month of the change, you should tell them in writing before they leave.
Otherwise, the employee may be entitled to sue for breach of contract, or resign and claim constructive dismissal.
You must tell the employee in writing about any changes to the written statement no later than one month after you have made the change.
Changing an employment contract involves a negotiation between the parties to meet everyone’s goals.
If you want to change an employee's terms and conditions of employment, you will need to get their agreement first.
But employees' responsibilities may change over time, and market conditions also remain in flux.