Unfortunately I don't think there is an easy solution. Because this page cannot be monetized, there really isn't any way for you as the dev to be able to track who previously bought the game through this page. All previous purchases are tied to this page's ID, and since it cannot be remonetized there's no way to link anything to those past purchases.
The best bet is probably Steam, and then you can go through the messy hassle of figuring out who previously bought the game and give all of them steam keys, while anyone looking to buy moving forward can buy through Steam. An imperfect solution, but a solution nonetheless.
EDIT: It might be possible to edit the current downloads to add a steam key option to the current paid tier, since those are still available to be downloaded by those who previously purchased. I don't know though because I've never tried to add more to an existing purchase tier before, let alone on a demonetized page.