As with website link above, fare rules say SDC must have same connection/thru-city, but I guess this isn't enforced very much. Fare rules mention a few exceptions where co-terminal changes are allowed.

D. SAME DAY CONFIRMED FLIGHT CHANGE AVAILABLE ONLY
FOR IDENTICAL ROUTINGS -SAME ORIGIN / DESTINATION/
CONNECTION / THRU CITY IF APPLICABLE . CHANGES
BETWEEN CO-TERMINALS ARE ALLOWED FOR SFO/OAK/SJC
AND LAX/SNA/BUR/ONT AND PAE/SEA.