Employers have a wide latitude in assigning employees to particular tasks or areas of responsibility. Employees are thus ordinarily required to accept instructions by employers even involving substantial changes to areas of responsibility. There is, however, a limit beyond which an employer cannot go without assuming a risk of breaching the terms of employment.
If the revised job duties involve what a reasonable onlooker would regard as a fundamental change, the employee would ordinarily not be required to accept that change. If the employer insists on the demotion and the employee refuses, a court might well regard the demotion as a constructive dismissal, equivalent to an actual termination, and award compensatory damages. In some cases this can apply where a promotion is offered, but that change is unacceptable to the employee.
The difficulty is knowing where to draw the line. If the employee refuses to accept new responsibilities that were within the employer’s rights, the employee would normally be seen as having quit or voluntarily resigned. The safest course in circumstances such as these is to obtain competent legal advice before deciding what to do.