Your position description and even your contract of employment can change without your explicit approval — if your silence (in the face of changes) offers that consent.
Most changes in duties aren't particularly worrisome. Indeed, some are welcome. And, I am not suggesting that every change to your position description requires your agreement. This is about changes that should have your explicit agreement — but don't get it. That's particularly true of changes that are out of keeping with the nature of the job for which you were hired, and, thereby, may affect your contract of employment.
If a change is proposed that you're willing to accept (but only temporarily), then it's useful to have an explicit understanding. And that usually means specifying an ending date or event. But whether explicit or not, make sure that the passage of time doesn't demonstrate what the words didn't — that the change was permanent.
If you're worried, have the Union assist in clarifying the duration of the change.
It's also possible to confirm, unwittingly, "temporary" matters as permanent. For example, if in an annual performance review you refer to your stellar performance of tasks unilaterally added by your boss, that submission might be taken to indicate your acceptance of those duties.
It is a stupid goose that listens to the fox preach.
—French proverb
