It may not exactly be the case any more - but part of this relates to system programming of the airport DCS system. The concept of "standby earlier flight with later confirmed flight" is fundamentally programmed into DCS with the ability of DCS to track your status and make updates on both your confirmed and standby flights based on whether you board or not the earlier flight. i.e. - you can be checked in for a later flight, with a confirmed seat and then standby for an earlier flight. You are essentially checked in for both flights (with your E-ticket status updated to CKIN). If you are confirmed and board the earlier, the later flight's seat will be released in DCS and you are essentially offloaded (unchecked) from the confirmed flight and the boarding pass cancelled (one thing that is interesting is that for same day standby, the system does not actually update the availability of seats for sale for the later flight in RES. Thus - if we think it will help AC sell more seats - it never really did because it actually kept you in RES as if you were booked on the later flight).

So - you can't standby for later flight because of the way the systems are programmed and because that is not actually how same day standby was supposed to work.

What you can do now however is actually make a confirmed change on the day of travel at the airport. I've read a lot of fare rules on this and some of them seem to indicate that you can make a confirmed change at the airport for any other flight on the same day as long as the booking class you have is available and your fare rules allow such a change. Here is some text I have seen. Here - you could technically have a P-Class fare and confirm J-Class later in the day - as it only says you have to have the cabin available and you can confirm the change for earlier or later. (see later for my example on YYZ-YOW)


--

PAX HOLDING CONFIRMED RESERVATION AND TICKETS ON
AC OPERATED FLIGHTS ONLY MAY CONFIRM EARLIER/LATER
SAME DAY FLIGHTS BETWEEN THE SAME ORIGIN/DEST./STP
POINTS FOR FREE OF CHARGE.
-SEAT INVENTORY MUST BE AVAIL. IN THE CABIN APPLIC
TO THE FARE CHARGED
-CONFIRMED CHANGE MAY ONLY BE MADE AT THE AIRPORT
ON THE DAY OF THE FLT.
-ANY AIRPORT SAME DAY CONFIRMED CHANGES WILL NOT
BE ABLE TO BE ADMIN. BY CALLING THE AC CALL
CENTRE OR BY VISITING THE AC WEBSITE.
-REGULAR FARE RULES APPLY FOR ANY CHANGES WHICH DO
NOT QUALIFY FOR THE AIRPORT SAME DAY CONFIRMED
CHANGE FEE
-SAMEDAY STANBY NO ALLOWED.

-
As for YYZ-YOW - I assume you are on Flex - The rules I can find say the following:

In summary - it says Same Day Standby allowed for free but same day confirmed change is $75. In that case - why doesn't one just book the very last flight of the day - (if cheapest) and then just always standby for the desired flight?

PASSENGERS HOLDING CONFIRMED RESERVATIONS AND
TICKETS MAY STANDBY FOR EARLIER SAME DAY FLIGHTS
BETWEEN THE SAME ORIGIN/DESTINATION AT NO CHARGE
OR MAY CONFIRM EARLIER/LATER SAME DAY FLIGHTS
BETWEEN THE SAME ORIGIN/DESTINATION/STOPOVER
POINTS FOR A NONREFUNDABLE FEE OF CAD75/USD75
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.
E. SAMEDAY STANDBY NOT ALLOWED EXCEPT FOR RAPIDAIR
FLEX TYPE FARES BETWEEN YOW/YMQ AND YTO
AS NOTED ABOVE.