ANA Experience Japan Fare allows same day standby for an earlier flight.

The more expensive (13000) foreigner fares also allow that, or even unrestricted change IIRC.

I have bought the fare and flew on an earlier flight one time I arrived at the airport too early.

>> I can't quite tell whether the discount fares require advance purchase

All spelled out on their respective official websites. Purchased prior to arrival in Japan (13000 yen varieties) , or 72 hours advance purchase (10000 yen)