I do this with some regularity, and have never been denied. Frequently switch LAX/ONT/SNA with SAN, and IAD/DCA with BWI. In fact, once I asked for and was granted IAH-EWR instead of my booked IAH-ONT. (I was flying LAX-EWR the next morning anyway and was happy to save my butt the additional mileage.) Didn't end up taking it, though, as I was flying with the wife who didn't want to drive home from ONT alone late at night---we were on the last leg of a return flight from Europe.