I just reserved a flight from BRU to YVR in H class and was surprised that there is no advance purchase requirement. I can wait up until the upgrade window to purchase the flight (even until day of travel). This basically allows unlimited changes until the day of departure. It seems strange that AC allows such a liberal policy on such a cheap ticket. If I booked through FRA I had to confirm within 2 days. By the way, everything coming out of Europe is jam-packed in economy around March 20-22. Good chances for bumps or OP-upgrade I would suspect.