You can do everything about it, it is your project page.
One cannot raise the donation and pay the difference. It would be full price again - and no guarantee that you raise price again in the future and make files unavailable again. That pricing option is not meant for setting the price of a game. It is only for bonus materials, like a soundtrack. If you sell the game files by price level you are basically stuck at that level. If you ever raise it, you are scamming all your previous buyers that thought they were buying the game.
Yes, that pricing option is very bad, even for bonus materials, and Itch should not offer it to begin with.
Easiest solution is to make this project page paid with minimum price and remove the price tags on the files. All your customers that gave you money will be considered to have purchased it and will have access to all the files that do not have a separatly set price. It is just like if they had bought the game at a discount.
So it heavily depends on what you intended to do, and what you promised the customers. You might even want to split off a separate project page and have different pricing options on both.
Right now, I see no hint whatsoever on your game page, what you actually offer. It is pay what you want. And not donate and support the developer and pay again, if the game gets released. So if I were to have paid what I wanted to access the game, I would assume that the game stays paid and accessible.