What makes you think Aeroplan books into Flex? Aeroplan books into their own inventory with their own rules. Check these.
Used to be that SDC was allowed for business class rewards.
Actually the conditions say:
Can I change my booking at the airport on my departure day?
Yes. Same-day confirmed changes at the airport are permitted to either an earlier or later flight on the same itinerary for a charge of C$75 per passenger holding either a confirmed Economy, Premium Economy or Business Class Aeroplan Fixed Mileage Flight Reward or Market Fare Flight Reward on Air Canada, Air Canada express, or Air Canada rouge. No additional mileage charges apply for changes made on the day of travel at the airport, but changes are subject to availability.