Originally Posted by eigenvector

Same day changes are possible, but we need to know your specific fare rules to know the applicable fees and restrictions. International P fares generally have free SDC. My impression is that the definition of 'SDC' as applied by Air Canada on domestic/TB is that it requires that all flights be operated by AC, but I don't have experience with this on international itineraries. This means that rerouting via ICN or HND as an SDC may be off the table.

Once you misconnect or misconnection seems very likely, pretty much anything is fair game.

My first thought is why not try to SDC to an earlier YOW-YUL flight to pad out your connection time? Then you don't need to touch the NH flight.