Effectively you want to offer a “paid” app with a free trial. To do this on our platform, you set the app as free and features/funvtionality is then unlocked via a free in-app purchase(IAP non-consumable) that grants access for a set period of time (days). When the IAP is purchased you will have a transaction with a purchase date, which you use to calculate your “paid app free trial” expires date (purchase date + trial period days).
at the end of the trial period, they then “purchase” your app using a 2nd non-consumable in-app purchase set to the price of your choosing to fully unlock/buy the app.
so while the app is free, features/functionality is gated behind these 2 non-consumable in-app purchase products. Note: if your app was truly a paid app before, you will need to honor their past paid app purchase before transitioning to a new business model using in-apps. Be sure to rev you applications version (Original application version)when you change business models so you can reliably know who paid for your app vs got it for free since moving to IAP.