Originally Posted by cringle

It's pretty clearly spelled out in more detail on the adjacent "How it Works" page. It isn't guaranteed:

Opt in for free upgrades to Main Cabin Select when there’s space available.
If you’re flying solo, you’ll have the chance to opt in for an upgrade while you book your flight online, through an authorized travel agent, or when booking through the Virgin America Call Center, and we’ll update your flight status with your new cabin (if available) up to two hours before departure.
If you’re traveling with companions, you can opt in to upgrade your seat by calling your exclusive Elevate Silver or Elevate Gold reservations line.
Since the upgrades are automatic, we can’t guarantee availability of any previously selected Main Cabin seats once you’ve been assigned a spot in Main Cabin Select.
If you’re upgraded to Main Cabin Select when you have already checked in for your flight, you’ll need to check in again online or at the airport to select your new seat in Main Cabin Select.