If you pay again, you buy again. The payments are not added together. This is very annyoing if a game has three or even four versions of a game on the same page. Like free, paid, extra, deluxe. You cannot "upgrade" to my knowlege. If this were a feature, you would notice this by the page not promting you to buy, but telling you that you would upgrade/donate, since you already own the game.
Individually priced files are a very bad concept on Itch. No one understands them. Not the users, not the developers. And the intended use case is almost never encountered or practical. https://itch.io/docs/creators/pricing#purchase-tiers-through-individually-priced...
Best use case would be for a free game to encourage people to spend at all to also get a fancy wallpaper or a soundtrack. But having a $ 10 game and said sountrack for $ 2 already runs into the problem that you cannot upgrade and buy the "dlc" for $ 2 if you already have bought the game.