I take you've already bought this ticket? On a lot of domestic flights, it's the same price for two one-ways vs a round-trip. If SAN one way is significantly cheaper than LAX one way then you can "accidentally" miss your connection and drive. Then since your return is actually a separate one-way, it's unaffected.

If your deal was indeed only if you bought round trip then you'd need an open jaw into LAX and out of SAN. Open jaws price as round trips though could be marginally different based on pricing difference between LAX & SAN.

None of this will help much now if you're stuck with a non-changeable ticket or unreasonably high change fee, but could be useful next time.