I don't think they even do it.
I tried for over six months to get an upgrade on a flight that was showing open/unsold seats up to the day of departure, even calling FD on a daily basis on the last week, but no joy.
It appears the days where Royal Wings (and Platinum Elites) essentially got a seat if it was open are long gone.
If you're departing from Schiphol or one of the other airports with "Day of Departure" upgrading, you may have to hope for that.