At the time of the original change, UA will allow origin or destination airport changes on irrops of up to 300 miles (use to be 250). This does not apply to voluntary changes.

YYZ-EWR is over 300 miles, (roughly 350)
YYZ-DFW is 1,200 miles

I have not heard the country issue but the fare rules for the ticket may not include a USA origin (even though there is a UA connection)

You can try now but you are asking for multiple exceptions.