hungarianhc

So I had an SFO-SEA itinerary booked for this weekend. My gf and I are going up to Vancouver. Last minute, I had to go to NY for work. So I just booked the following:

SFO-JFK -- LGA-DEN-YVR

I want to cancel the first portion of the ticket. I have had this situation occur MULTIPLE TIMES before, and they always can cancel one leg of my trip. Is there a specific policy I can cite to have her do this for me? Or did I just get lucky before? If so, I'm really screwed because I need the return flight from SEA-SFO at the end of the weekend... and i know if i just "miss" the first portion of the ticket, it will cancel the rest of the itinerary - help!