AC*SE and Empress are indeed correct. As long as the flight is not oversold, the Aeroplan agent can take a Y or J seat (depending on status) and send it to the route controller for conversion to W or D class. No direct contact with the route controller is needed except for agents to send it to them for the actual conversion. And sorry Gold Flyer, but I know of no route controller for *A partners that I have access to.