On appointment, there's an appointment letter. That letter and its acceptance usually constitute the contract of employment. The terms and arrangements of the contract are usually circumscribed by a collective agreement (union agreement, it's to be hoped). Usually the collective agreement spells out pay rates and conditions — and amongst the conditions are generic position descriptors.
Generic descriptors are designed to classify the levels of work and tie them to applicable rates of pay. To do this they distinguish, for example, the academic role of a lecturer from that of professor, or a professional staff classification that supervises others from a classification that doesn't. (Because many of the features of a particular descriptor are common to all, the duties that comprise the differences are usually referred to as distinguishing duties.)
But enough foreplay: Within all this, then, are the (anything but lowly) position descriptions (PDs).
While there are often commonalities amongst PDs, each PD is intended to address the actual work of particular a job. (Sometimes, however, more than one individual does the same job — in a phone bank, for example.)
Contracts of employment can change only by agreement or by acquiescence (a topic for another time). Union Agreements (and the generic descriptors) change only through negotiation. PD's, however, can change over time without the agreement of the job holder. Any such changes must live, however, within the bounds (or "ambit") of both the contract of employment and the applicable Agreement descriptor. So, if you were hired as an accountant, you cannot be required to be a driver.
Most PD changes are evolutionary: A new computer system, for example. Some are more abrupt: Changed reporting lines, for example. But because management has some prerogative to make changes, supervisors occasionally seem to believe that PDs are infinitely malleable. They're not.
So, how malleable are they?
Every position is different. When PD changes are proposed, contact the Union — you'll work it out together.
There are 4 kinds of Homicide:
Felonious, excusable, justifiable, and praiseworthy.
—Ambrose Bierce (1842-1914)