Once you buy a game you have access to the files regardless of what the project price changes to in the future. There is one exception to this, if you use individually priced files then the price of the file controls who has access to it.
Viewing post in Question regarding minimum prices
A free download does not give ownership of the project page (unless it was granted through a 100% off sale).
You can read more about it here: https://itch.io/t/41926/who-can-download-my-game-when-i-convert-it-from-free-to-...