It depends on the specifics of the fare you purchased. You will not get a OAK/LAX (WN competitive) changed to a SFO/LAX without some additional charge - also the city change IS a reroute for ticket issuance. There are PFC differences. The co-terminals are SPECIFICALLY listed on each fare basis published and may not be consistent between different fares and different markets. (ie, BUR/ONT/LAX/SNA may all be the same on a particular IAD/LAX farebasis, but not out of SFO.)