not a routing us-canada-us, but nw had irrops years ago when i flew pdx-sfo-ams. they rerouted me pdx-yvr-ams.
i was already checked in, and when i got to the gate, they decided that plane was going to sjc. they told me i would fly a day later. i insisted they re-route me and they did. had to pull the luggage, and re-check in with alaska.