The problem you have is the lack of detailed knowledge about *ONE* t&c's by the phone agents. Far more common post CV19 with most(all?) airlines.
Changing LHR-SEA-SAN to LHR-SAN (non stop) would be a reissue.
https://en.wikipedia.org/wiki/San_Di...tional_Airport
https://en.wikipedia.org/wiki/Seattl...tional_Airport
Certainly is past times could book/change prime-codeshare flights on *ONE*, without cost. The earnings on A D L fare buckets could be different depending on the marketing carrier(flight number) and frequent flyer program. Was therefore possible to increase ff earnings by a change prime-codeshare flight.