if you ask for the upgrade at checkin, early enough it should work better... and if they give catering as an excuse, go to the ticket office and ask nicely "excuse me, is it possible to buy a full C ticket for the next flight?" and I am pretty sure that there will be no problem with that if there are seats available in C and the flight is not much overbooked
And that they will even manage to find a meal for your 6000 USD

but for 20000 points, i think they just don't want to bother and go the extra mile...