I use Air NZ as an example, it is not unique.
You can buy a flexible fare, and change e.g. the date of travel maybe for a fee and price difference. But if you look at the terms and conditions on the Air NZ website:
" Different ticket prices
If our standard fare for your new flight is higher, you need to pay the difference."
So, what is Air NZ's "standard fare" and what is the "difference"? I feel it may not the web fare for the date we wish to move our booking to, and nowhere can I find a definition of "standard fare".
In @sbiddle's writings linked to in another post referring to AirNZ, he writes "Each of these fare classes is set at a fixed price which is published and publicly accessible." https://traveltalk.nz/news-opinion/airline/air-nz-cuts-entry-level-domestic-airfares/ It that is so, then it is not in a published form that I can find, and I spent more time that I should looking. Are these the "standard fare"?
I ask as someone asked me what would happen if they needed to change their dates of travel.
As an example, but let's not get in a Jetstar discussion. A few years back I was crooker than a crook dog, and had to move a flight on Jetstar. I'd paid $280, and looked to change the flight to 4 months later. I couldn't do the change on the web, so phoned up to make the change. I was charged the difference on a fare of $344, even though the internet prices were considerably lower (cant remember exactly). Got told that no, the web price is not their standard price and that I would have to pay the difference between what I paid and the standard fare. That fare was in excess of any of the web fares visible on that day.