Maybe one idea: Since the property for the installed checkbox is a string and not a bool anyway, you could give us the option to choose to display it as checked for games that have updates available?
Ehm, NO?!, I'm one of those who never keeps a game installed after playing... mainly due to the number of VN's I have in my list. I would need a separate HD to keep all of them installed. Your suggestion, if honored, would make it look like I still have it installed, thus not being able to separate them from the ones I actually do have installed...
Had to adapt already to the 180degree behaviour change from 8.5 to 9.0, this would complicate things even more... Checkboxes are booleans, so it should be either on or off. That Willy "abuses" it internally to keep track of last installed version makes no difference, outward it still should act as bool.
He's already considering adding an indicator in front of the title and changing the color highlight... me thinks that should be more than enough to alert you that a game has an update.
Plus, the meaning is exactly what you're triggered to think, you may have the old version still, but the update isn't installed. Which is what a cleared checkbox reflects.
Sorry, I don't mean to sound blunt, but like I said, I just think the options shown in the preview vids would be the best.