Hey @Fahed Masood , there are a few ways to make this happen. Here are two ideas:

• Use the CustomerInfo object that you can get from the SDK (e.g., on iOS, Android), and check the firstSeen date – if it’s before the date you introduced subscriptions, grant the customer free access, otherwise, show the subscription screen.
• Grant promotional access to all existing customers. You can do that via the RevenueCat dashboard (only if you don’t have too many existing customers), or via the REST API. I would go down this path only if the number of existing customers isn’t too large.

For more information, you can also read this article (although it’s more focused on converting an app that was previously paid up front): 

Converting a Paid App to In-App Subscriptions – RevenueCat

And how do you do this if your app has been used for three years before you install RevenueCat? In other words, none of the users who downloaded the pay-to-install app have ever been seen by RevenueCat

@luke-pighetti check the blog post I linked above. You can use the original app version ID as a filter which you can get from the app store directly, this tells you the first version that a user had originally installed of your app. If that version is less than the version where you went from paid up front to free + subscriptions, you can grandfather them.

https://www.revenuecat.com/blog/engineering/converting-a-paid-ios-app-to-subscriptions/