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."

https://www.airnewzealand.co.nz/bookings-faqs#cost-domestic

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.