I'm a little unclear on how payment works. 

Let's say...

1. I release a demo of my game with a minimum price of $0.
2. Someone decides to donate $1 when they download it.
3. Later, I release the full version with a minimum price of $5.
4. That same person comes back to download the full game.

What happens?

Do they have to pay the full $5? Do they just pay the $4 difference? Do they get the full game for free because they already paid once, even though it wasn't the new minimum?

I read the FAQ page (https://itch.io/docs/creators/how-buying-works) and I think what I expect is that itch.io tracks the amount they've spent on this product to date, so the user must pay the difference. I'd just like to clarify though. Thanks! :)