late minute award availability on ANA using United miles

Hi everyone,

I have booked an award reservation on ANA NH1009 JFK-NRT on 12/21 using United miles. I'm interested in switching my flight to NH9 (earlier departure) on the same day if an award seat becomes available. Is there a cutoff when I can no longer make the change even if a seat becomes available? Can a change be made on the day of the flight? Can ANA check-in or gate rep make any change to bump me to the earlier flight (NH9)?

Thanks for your help!
kunia