Economy fares from Point of Sale NZ in classes H,M,B,Y are flexi, if you are given the option to make it "flexi" it bumps you up to H class (the lowest of the flexi classes). If you decide to make a change to another flight at a later date, a fare difference is applicable if selling above H class at the time of change. If the new flight is still at or below H class then no fare difference will apply and you can change at no cost.