Why are you wanting original routing credit? You will still earn 500eqm/1000RDM on the flight you've been rebooked. That's the same that you would have earned on LAX-SAN. Like a previous poster stated, you might get lucky and get a COS bonus on the LAX-CLD if they rebooked you in Y (which is often times the case).
UA is not going to give you credit for the original route and the new route.