I only have experience in changing destinations in cases of IRROPs, VDB or schedule changes. However, in your case I would look at the specific fare rules as I know UA considers SJC-SFO-OAK as the same destination, likewise LAX-SNA-ONT-BUR. There are some other cities for this as well (i.e. MIA-FLL off the top of my head). If this is in your fare rules, you might have a fighting chance with an agent.