What I understand, but I can't officially confirm.
If someone donated money for your game while it's free, it doesn't matter how much it is, as long as it's above the minimum you selected, they are considered to have "bought" your game.
For example, if your minimum is 3, and someone donated 5, that person can't access individual files that cost 10, but Itch considers that person to have bought your game.
If you change from free to paid tomorrow, and remove the individual price per file, it doesn't matter what price you put on your game, that person is now considered to be the "owner" of the game and will be able to download the game.
If you create a second page, you can manually send keys to people who have only paid 10 or more.