Originally Posted by
pilot007
Flying later this week. I've tried to get this out of CWT, but they have no idea what I'm talking about when it comes to SDC. They just quote me the negotiated fare rules, which permit changes, but doesn't mention anything about same-day changes. Guess I'll try my luck later this week, and hope to get to SE100K next month so that I can at least email the concierge in the future!

As per ABG, they (CWT) would not care or necessarily know about the SDC rules on their side of things as that wouldn't be negotiated per se -- did you specifically ask them for AC's fare rules associated with this booking (so that you can read it yourself)?
In any case, as alluded to upthread, it looks like your restrictive Z fare class is not exempt from an "at the airport same day of flight" change fee.
Are you sure the subject flight's CWT-booked fare is identical to previous ones, or possibly slightly different? If it has been the same throughout, then consider yourself a beneficiary of good concierge service.