Originally Posted by Jasper2009

Correct.

This information is even published on the AC website:
https://www.aircanada.com/ca/en/aco/...s/request.html

-> Between the time of reservation and up to 24 hours prior to departure

For eligible customers travelling on International flights with a Latitude (booking classes Y and B) or Premium Economy Flexible (booking class O) fare, upgrades are based on the availability of “P” booking class at time of request. For eligible customers travelling on North American and Caribbean flights with a Latitude (booking class Y, B) or Premium Economy Flexible fare (booking class O) fare, upgrades are based on the availability of “Z” booking class at time of request.