CARs 700.26(1) states, "An air operator shall not allow a flight crew member to begin a flight duty period if, before the beginning of the period, the member advises the air operator that they are fatigued to the extent that they are not fit for duty."
But what happens if a flight crew member says they are not fit for their assigned duty, but could do a different duty? The CARs is not outline this situation, but section 4.12(5) of the Advisory Circular (AC) No. 700-047 says, "If a flight crew member were to advise the air operator that they are unfit for the assigned duty but they are fit for a different duty, it would be reasonable for the air operator to reassign the flight crew member to that duty."
This means, in theory, an air operator could give a different, presumably shorter, flight duty period.