The ability to change to another flight free of charge on a flexichange/flexirefund ticket on the same day opens midnight NZ time, not T-24 as some are lead to believe.

I’ve waited til 12:01am several times and changed my flights in the app. It’s a super easy process, it just brings up the usual flight change option but all flights on the same day for the same routing are “change from $0”. Hope that helps