Interesting question. With respect to
can I change the departure airport to OAK or SFO during check in?
the answer is almost surely no (though I haven't seen the OLCI page in three years).
If you call you will most likely be asked to pay the fare difference (if not exempt by status). It could be worth trying at the ticket counter. If you are told no there, you could try again with the gate agent.
With IRROPS (even beyond just a flight cancellation), you can even change (at the airport) to an airport that is not a co-terminal.
Of course seat availability is always a potential issue.