From Reddit
As per the example in the Reddit thread. The user wanted to book a flight at 6PM which costs $350. The 9AM flight however is $109. If you book a Flexitime Fare for 9AM, you can then change to the 6PM fare for free saving you $241. The caveat is that there may not be room with the later flight so that's a risk.
The key is to hop on at 12AM on the day of your flight and change it ASAP. Anyone tried it?
Think you have the wrong alternative store, should be airnewzealand.co.nz.