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.