I had a similar experience but with a nasty twist.
I tried to change my flight date (2 months out) HK to AKL and they were adamant there was no availability. The interesting thing was EVERY other airline I went to did have seats available. I ended up buying another ticket from a different airline. Cathay Pacific then threatened me with the no show fees. I calmly informed them I would never fly with them again so I was not going to pay the fees.
Appalling, arrogant, rude and I suspect dishonest service. Recently a colleague encountered a similar incident with Cathay. We came to the conclusion they were trying to “sting” us up to a first class upgrade as we were both encouraged to upgrade (at full price of course).