If you are lucky, you get a call asking which ticket to cancel. If you are unlucky, they auto-cancel the correct ticket. If you are really unlucky, they either cancel the wrong ticket or both and you have then noshowed for your onward trip and all of that gets cancelled.

While you might be able to get everything fixed, it will take hours on the phone (much longer than the SFO-SEA flight) and a lot of heart ache.