I've made plenty of changes to an award that contains an AKL-YVR segment in C on NZ and there has been zero award inventory on that flight since I booked it back in December. I've made changes to both UA flights and non-UA flights on the itinerary and while I've had agents make up their own rules and had to hang up and call again, no one ever said changing one flight requires award inventory on all of the existing flights that aren't changing.