It's always helpful to read the first post of an FAQ thread.

From post #1:

How can I determine available upgrade inventory?
What does "I space" mean?

Air Canada will not simply allow upgrades into every available business class seats at any time. Obviously they would prefer to sell those seats. So the number of upgrades is limited. It is possible to determine if there is upgrade space available on a particular flight.

The first method is to use aircanada.com. When you are selecting flights you can check the box beside Display only fares eligible for upgrade. Enter the number for the certificate you want to use (number are listed above). A blue arrow will appear beside any flight with upgrade space cureently available.

Another method is to use a third party fare class availabilty tool. The upgrade fare bucket for upgrade space is I. (Hence you will often see the phrase "I space".)