The contract of employment may contain an express term allowing your employer to change your duties.
There may also be an implied term permitting changes, for example where new technology has changed the nature of the job. Otherwise the employer would need to get your agreement to a change in your duties in the same way as any other change would require consent.
Any changes to the contract must be notified to the employee within one month.
Just One More Click: Doggy drama about staying safe online at work.