I'm considering booking a NZ WLG-ZQN flight on flexichange fare. Complete newbie here to the flexichange fare (haven't flown NZ domestic in over 10 years).

Fare rules state "same day changes free of charge between same origin and destination, subject to availability".

• Does "availability" mean any seat is available on the flight, or does it mean only a subset of the seats (i.e. a certain booking class code)?
• If I was booked on WLG-ZQN, but a WLG-CHC-ZQN is available on the same day (assuming this was offered as a married segment), if I wanted to change to that indirect flight option, could it be done without fare difference?
• Can a change to another flight on the same day be made after the check-in for the original flight closes or after the departure of the original flight, again without fare difference?

How can changes be made on the same day, if desired? Is it done online, over the phone, at the airport or both? Is there a fee if the change is requested at the airport or over the phone?

Thanks for all your help!