There is a section in your library that contains all the "owned" games. If you download a game for free, it will not appear there.
If you pay something, even for a free game, it will appear there and stay there. The mechanid behind this probably is the fact that one does not even need an account to buy on Itch. People get a download key. https://itch.io/docs/creators/download-keys#download-keys-and-urls/file-access
So anyone that paid anything for a project should have such a download key and be unaffected of any minimum price changes. This goes for bundles as well.
But I do not know how a bundle price is compared to those individual priced files, or how discounts are considered. If you buy a 2 item bundle for 10 and there are individual files for 10 on both projects, are those unlocked, or is the paid price distributed over the 2 projects. And what if you buy at a discount. General file access is clear: you have a download key.