The only thing I've ever been able to do in this situation is convert to a one-way and receive a voucher for the difference. I've actually never known anyone to be successful doing what you describe - so that is pretty amazing, that you've succeeded in the past.
One thing you COULD try is, if you booked the new tickets within 24 hours, cancelling them, booking to SFO instead, flying SFO-SEA, then taking the train, the bus or a car to YVR from SEA. It would be end-to-end ticketing, and I don't know that it would necessarily save you money, but it depends...