exerda

By the rules, no. That said, you may get an agent at SAN who is receptive and who will make the change--or he or she may stand pat and insist on the $200 at a minimum (plus potentially the fare difference).

I have actually done this before in the opposite direction--e.g. showed up early at LAX for my flight to OAK, and asked to switch to LAX-SJC (LOL, back when UA actually flew those!). The GA made the change without batting an eye. But I've also had to pay the "service fee" when misreading fare rules on an IAD-specific fare (although it had the "standby permitted from DCA" text in it, it *also* had text saying fare was valid from IAD only).

If there are irrops at all with either of the CLD-LAX or LAX-ORD flights, then you have a much better chance of it.