Air Canada uses 2xxx flight numbers mainly to denote 'rescue' or 'replacement' flights in the event of cancellation, diversion etc, or - my best guess in your case - the original flight is so extremely delayed that virtually anybody aboard will have blown their onward connections.

The aircraft operating your flight was 5 hours late arriving at PBI, and ExpertFlyer's status notes show "UNSCHEDULED MAINTENANCE - DAMAGE DURING GROUND OPERATIONS" as a potental cause.

I won't speculate on your particular flight without knowing the facts, but in terms of what might prompt a message like that, this could be something on the order of "a luggage truck bumped the aircraft and they had to do a safety inspection" or "the aircraft blew a tire and it took a few hours to get the replacement put on", or any of dozens of other issues where the flight can't take off until the inspections have been re-run and the checklist has been met.

Again, I'm not saying that's exactly what happened to your specific flight, only trying to give you an idea of the kind of thing that might lead to this outcome.

In this case, one key benefit to cancelling 1237 and replacing it with 2237 would be to trigger auto-rebooking for anyone aboard with a connection onward from YYZ.