It appears you created an entirely different product page, which is why they would not have access. A purchase is tied to a specific product page.
Regarding contacting them, you can use the built in email tool to send an email to all buyers/donators/etc. I think the easiest way would be to send out an email saying something along the lines of: “Thank you for paying for XYZ, I’m switching over to paid, if you want a key for the new page please reply to me here <your email>”
You may also be able to access the buyer’s email address directly from the purchase details dialog on your dashboard. If it’s only one person you could email them directly.
Hope that helps!