Your entire ticket would be cancelled, and you'll get the value of the ticket as a credit towards another flight for 1 year (not a refund, it would be credit on Air Canada.com), minus a $300 cancellation fee.

Your options I'm afraid are limited. Same day changes are only valid for earlier flights.

So really it's a case of cancel the ticket for $300 per person and then rebook. But the rebooking at a late stage will probably be much more expensive. Pay the change fee, which should be $300 per person plus fare difference, which is pretty extortionate for a YHZ-YUL fix, but that's the fare rule you agreed to when booking the ticket.

Or third, do everything in your power to be at the airport on time.