The first (and last, probably only) time I've ever booked a "C" class ticket on aircanada.com I was given a C class ticket then assigned a seat in the bowels of the economy cabin. When I called to inquire about it, the agent said they would check to see if there was any "upgrade" space available. There was, so she upgraded me.

In the mean time, I cancelled the ticket, rebooked in M class, and then upgraded, as in, the old fashioned type upgrade, where you get upgraded from one class to another.

Given a whole series of other problems with booking on-line, I've abandoned the effort altogether.