Looking at the CWT fare rules may not be enough for something like an at-the-airport SDC...

I agree with the above poster though that if the AC fare rules you were previously booked under did not allow free SDC and required a change fee (which is likely for restricted fares such as P and Z), then the concierge did you a favour...

(and it's nice to see that a non-status (at least, non-SE) international J-cabin paying customer actually does get concierge service at the airport. )