I would like to increase the buying price for one of my games, but I don't want people who have already bought it to suddenly lose access to it.
Will the previous owners still own the game if the price increases?
When someone pays for your game they get access to it via a “Download Key.” Even if you change the price later, either up or down, anyone who has previously bought it will still own it and have access to it.
There is one edge case for this, for individually priced files. Those are limited by the actual purchase price.
You can learn more about “Download keys” here: https://itch.io/docs/creators/download-keys