I don't believe that AC has a formally stated long term policy about this, but over the past few years eUpgrades that have been issued after November 1st have an extended expiration date.
You can see the current year's expiration date policy in the eUpgrade terms & conditions (
https://www.aircanada.com/ca/en/aco/...onditions.html):
eUpgrade Credit validity period
All credits are valid through the Altitude benefit year, unless otherwise specified on your eUpgrade account. The 2019 benefit year runs to February 29th, 2020. Furthermore, all credits earned on or after November 1st, 2019 will be valid until February 28th, 2021.