Short answer - no.

You can cancel and then ask the call centre to off-load you - this happened to me last week. I was in YYC checked in to YVR and then YYJ. The YYC-YVR flight went mechanical and was delayed 2 hours. I then called the call centre (couldn't do it at the airport as I was on a NA Unlimited Pass) and they cancelled the YYJ flight and offloaded me from the aircraft.

However, for a seat change - that has to be done at the airport.