I did it about half years ago for some reason. I thought it will work like "donate another 5", isn't it? Cause I'm not really sure.
Unfortunately not. After some research, the way the payment works with files is like tiers, and in my case, I paid $10; I only have access to files at the $10 level or below. If new files come out at $15, I can't access them. And it doesn't let you add to your price, so it would be $15 more to get to the tier. It's one of the problems with having multiple files at multiple prices on itch. It's not you; it's a problem with their system, and it's really confusing. Their system wasn't really designed for multiple versions at multiple prices. It's like multiple editions of a game; the core game doesn't change price, but you pay more and get a sound pack or art pack kind of thing.