Typically, this is what will happen - you will appear on the UPG & StandBy list simultaneously (if you look at the monitor).
However, you can't clear UPG until you are a passengers on the flight. Assuming you are on STANDBY list to begin with - they must clear the UPG to clear up seats in Economy. If you get seat assignment and still a seat in 1st class you might make it but typically they have cleared all UPG (including Silver) before you are assigned a seat so just hope you get an EC seat.