USH- Dated tickets upgradable?

[Kiwigirls]

I'm planning to buy tickets online to USH for a specific date to save a bit of money. However the day we are planning to go is also the day we are arriving in the country (very early in the morning).

If, touch wood, the flights were substantially delayed then I would probably want to change our plans around and go a different day. Would I be able to pay the extra at the gate to change them to 'any day' tickets? Or would we just lose them altogether? Thanks!

 

[SMD]

The website says that you lose your money if you don't use dated tickets on the specific date.