AC's problem is station by station inconsistency vis a vis upgrading. I have received op upgrades at check in at some stations, and on the plane at others [Concierge comes on board with new boarding pass]. Sometimes there is a page in the MLL. Least effective is at the gate, since most elites only get down to the gate as the flight is boarding, and may have missed a call on ops.

Empress is correct to note that AC sells advance booking ExecClass on many routes, and these come from C inventory. As do the Diners and Aeroplan [non award] two-for-one tickets.