During the life of their employment an employee can become attached to the job they are performing. This can lead to an employee believing the details of that job are one and the same as their employment contract. Consequently they form the view that any changes to those duties or details can only occur with mutual agreement.
But almost universally, the tasks or duties identified on recruitment or thereafter are not implied terms of the contracts of employment such that a change of duties required by the employer is a repudiation. This fundamental aspect of the employment relationship led an employee into error when she claimed constructive dismissal.