Hmm, seems KVS answered my question for me:

AIRPORT SAME DAY CONFIRMED CHANGE FEE-

[...]

FOR ALL OTHER FLEX TYPE FARES ONLY-
PASSENGERS HOLDING CONFIRMED RESERVATIONS AND
TICKETS MAY CONFIRM EARLIER/LATER SAME DAY FLIGHTS
BETWEEN THE SAME ORIGIN/DESTINATION/STOPOVER
POINTS FOR A NONREFUNDABLE FEE OF CAD75/USD75.
NOTE-
A. SEAT INVENTORY MUST BE AVAILABLE IN THE CABIN
APPLICABLE TO THE FARE CHARGED.
B. POLICY APPLIES FOR PASSENGERS NEXT OUTBOUND OR
INBOUND ONLY. CHANGES CANNOT BE MADE TO INBOUND
SEGMENTS BEFORE THE ORIGINAL TICKTED OUTBOUND HAS
BEEN FLOWN.
C. CONFIRMED CHANGE MAY ONLY BE MADE AT THE
AIRPORT ON THE DAY OF THE FLIGHT AND PRIOR TO THE
ORIGINAL DEPARTURE TIME.
D. REGULAR FARE RULES APPLY - CHANGE FEES/ADCOL-
FOR ANY CHANGES WHICH DO NOT QUALIFY FOR THE
AIRPORT SAME DAY CONFIRMED CHANGE FEE.