Qantas is correct.
Original itinerary home is Zone 10 - 55,200 points
New itinerary home is Zone 7 - 37,600 points
That's a difference of 17,600 points minus 5,000 change fee equals 12,600
For 2 people, the refund is 35,200 points, perhaps that's what the agent meant when you were told 35,000...