On the game’s page, before a user makes a purchase, there is this copy:
In order to download this game you must purchase it at or above the minimum price of $X USD. You will get access to the following files:
And then it lists all the files available for that game. That means if the user pays that price, they will get access to all the listed files indefinitely. This includes all files added later, especially updated versions of the game.
The only exception to this, is if you set a specific price, by selecting on the desired file the following option:
Set a different price for this file
This will require users to pay separately a price, which should be more than the price of the project. When selecting this option, the following text appears:
Minimum Price
The minimum price for a file should be greater than the minimum price for the entire project. You should only set this if the file’s minimum price is different than the project’s.
As far as I’m aware, if you delete such a file, and you upload a new one with the same price, there is no way for Itch to know if you intent the new file as an “update” to the old file, so it will require all users to pay for the new file. The only exception to this is if you use the tool “butler” to upload an update for an existing file (as in, they have the same “Channel Name”). In this case the file is updated, and buyers still own it’s updated version.