There are many many examples of this. Beats Music was one of the more recent high-profile ones that comes to mind (for shifting from an offline-only pay model

There are many many examples of this. Beats Music was one of the more recent high-profile ones that comes to mind (for shifting from an offline-only pay model to in-app payments for subscriptions). Most of the team productivity apps, as another example category here, that have a premium tier don't offer them in app (Asana, Trello, etc). There are particular rules to how you do it. You can't promote it in the app. The app has to have some functionality for non-subscribers. You can't use the itunes subscription SKU functionality to delivery your subscription content (obviously, I hope). You should also consider joining the Enterprise app developer program that Apple offers if you want to by pass some of the above issues. Not to be overly pointed, but it doesn't seem like you have done much homework here. I recommend you get more familiar with the market you're about to enter.