Update: I chased GoToGate on this, and after consulting his supervisor, the agent called QR on the other line while I was on hold (thanks for that tip, mlqsko).

Qatar are having to make some kind of internal request for business cabin availability across both the OSL-DOH and DOH-BKK segments, which could apparently take a few days. Interestingly, if it comes back as refused, GoToGate informed me that they'd absorb the cost of a new ticket in J (but also that they'd hold out a while for another schedule change before doing so).

I'm tracking the booking using My Flights. Guess I'll call back next week if I don't hear anything.