I now have an App in the App store that is normally sold, which contains 6 IAP Noncosumable items, is also in normal state and has been purchased by many users. Now I want to change this App to be sold on an annual subscription basis, please:
Future versions of your app need to check what version of the app users originally bought, i.e. the old paid version or the new free+subscription version, and behave appropriately. There is a field in the App Receipt that you can check to determine this.
It is recommended changing business models is aligned with a new app version. But if moving from non-consumable IAP's to auto-renewable subscriptions, be sure you're app has the proactive restore best practice implemented so your app on launch checks the customers purchases asap and then your app react accordingly.
See our session here: https://developer.apple.com/videos/play/wwdc2022/110404/