To do what you posted above, you will need to short-check your bags to YYZ, go through Canadian immigration, pick up your bags, go through Canadian customs and take the train to Terminal 3 to check-in for your flight with CO. There will be no other way to retrieve your bags and leave the US Preclearance area in Terminal 1 since you will not have gone through Canadian formalities as a connecting passenger.

AC will not check your bag to EWR in Tokyo when your ticket is NRT - YYZ - DFW, even if you show them your other ticket.

Thanks for all the info, though it appears that my intended plan may not be so straightforward. Based on what I have learned here, two questions:

1. Does anyone have experience with AC and short-checking bags?
2. If I book another itinerary YYZ-EWR with AC (or UA) instead, and therefore do not have to change terminals in YYZ, how does that change the process? Presumably, in that case the US Preclearance route is still the correct one to take, and I get reunited with my bags along the way. Can I recheck them at a rechecking desk after customs to EWR based on the new itinerary?

Option 2: You'd somehow need to have boarding passes already. But there is a desk after US Preclearance, before security, where you may possibly be able to get your bags re-tagged with your new flight.

If you don't change the ticket, but buy a new AC ticket to EWR, and try to get a new bag tag/BP after US preclearence, an astute enough AC staff member may wonder how you got there in the first place to recheck your bag, and proceed to look at your ticket from NRT. If they do so, AC will have the right to charge you any fare difference between the fare you paid to DFW and what it would have cost just to YYZ (and I presume they would charge the walkup fare for that).

I don't have experience trying something like this but they can charge you, as the CoC you agreed to when buying the ticket states the fare is valid for travel between the original departure and destination - which YYZ is not. They may also choose to ignore it.

Your best bet is if you can have your bags short-checked, but that is a big if - luck would have to be on your side.

The sure way, since no one really knows what other ways might work, is to just suck up and pay the change fee and whatever the fare difference is. Another way would be to just fly to DFW and book a sepearate OW ticket to EWR from there. Yes, this will take longer, but in the end, it may actually be cheaper then a ticket from YYZ and is certainly sure to work.

I recently did ATH-YYZ-ORD on AC so I went through pretty much the same process. I don't see how this will work on your own. You will need someone to make an exception for you.

The airport is essentially like 3 places. Canada, USA and everywhere else. When coming from elsewhere you will see signs for Canada and USA.

To get your bag you are going to have to go to the USA. After claiming your bag is probably going to be your best bet to enter Canada. At this point you have not cleared any security or immigration. You would have to actually walk backwards towards the Canada entrance though there is not actually any security of any sort to stop you. Somebody will be watching the baggage claim though. You probably would have to explain to this guy thatyour bag is checked to DFW but you changed your mind and want to skip your flight and enter Canada. It might work.

If you continue after the baggage claim as you are supposed to you will hit US immigration and then customs. From there there will be a bag recheck but I see no way to get a boarding pass at that station. All they have is a bag recheck but the bags are already tagged. After that is security and then the gate area. There is no way to leave the gate area. Once you clear US immigration its as if you are in teh US. You can't leave.

Your best bet is to change your flight to DFW. Even if you could change it to the next day you could then enter Canada. Otherwise you can take your chances short checking to YYZ or having the baggage claim guy allow you to backtrack towards canada immigration.