Correct me if I am wrong.
But if you paid any amount at all, you will have access to paid files, even when you paid (or "donated") while the files were free and the project was changed later to paid. (Unless it is "individually priced" files). Basically it would work, as if you got the game at a discount, only that the discount ratio is determined when a free game changes to a paid game.
So accordingly, a game would change category in library as well from donations to purchases, if the project would change.
At least that is how I understood the faq.
I guess it get's complicated with individually priced files on free games. You got access to files by donating. But since the project is free, it should count as a donation.
In other words: games that appear in browse under free games show up in the library under donations. Games that appear under paid show up in regular purchases.