Itch should overhaul or abolish this feature. It is confusing and does not work intuitively. People cannot even increase the amount later. Buy game for 10 and later decide to also get the bonus soundtrack at indiviual price of 15? You can't do that. You would need to buy for 15 again.
You do not set the price of the files, no matter what you think or what is written in the faq, or how this feature is misleadingly called.
Users buy a project. Or access to that project's paid files to be precise. They gain so called ownership. And this does not change, no matter how they obtained ownership. At a discount, at a lower price, a higher price, with a promo key from the developer, whatever.
And there is optionally a method to make files only be available, if the actual paid price currently is above the "individual price" of the file. But it is not a price. It is an unlock tier. The amount you paid does not care, if those files exist or not, or how many of those files there are or if there are several tiers. You can't buy those files individually. They are not priced. Access to them is unlocked.
Switching such a project to a non zero minimum price should work, since any amount at all that was paid will grant access to all the project's files - except those individual files.
It is better to create two projects, instead of fudging payment options into one projects. Like the soundtrack I mentioned as an example. I do not even know, what happens when people buy on a discount. Or in a bundle. Or in a discounted bundle. I asked in community, but never got an answer.