Update app without "Offers In-App Purchases"

Changing business models is fully supported, the important step here is your rev your app version with your business model change. From a technical pov, the original application version in receipts or with appTransaction will give you an indicator of what app version the customer has first acquired and of course there are the in-app purchase transactions to dictate their subscription status.

The key for your app to adopt the "Proactive Restore" best practice as it ensures your app does a series of important checks immediately on app launch (at least the first time) to determine the customer state which will inform your app on what user experience to deliver. https://developer.apple.com/videos/play/wwdc2022/110404/

Be sure to consider your past and loyal customers and ensure they continue to have a great experience, maintain functionality and/or have a thoughtful upgrade/transition experience.

In your scenario they already have the app (free) regardless if they are an active subscriber. So will want to decide if you will stop the subscription entirely (remove it from sale) or let it remain for past customers that have the older version of the app (when it was free). If you stop offering the sub entirely, then you would use a non-consumable IAP to effectively gives your users a path to buy it who got your app while it was free.

Say you update your app to version "3.0" when you make it a paid app. then your app upon app launch your "proactive restore logic" checks the original_application_version.

• If equal or greater than "3.0" , then just turn on features as you know they paid for it.
• If it's <"3.0 then you know they own the app and they may be a subscriber. If they are an active subscriber turn on features
• if not a active subscriber then you would merchandise them your IAP options: either continue using subs or you just move to the 1-time purchase of a non-consumable.