Earlier quoted context omitted.
Why can't you update the app and hide most of the new features behind an in-app-purchase paywall?
New users who purchase the updated app would have to pay again to get the latest features.
It works like this: developer releases the app for, say, $10. People buy it and use the basic features for a while. So far there's no upgrades available. Developer codes up a premium feature and updates the app in the store. Existing users can use the app as-is with the basic features that came with the $10 price tag. Existing users can also make an in-app-purchase for, say, $5 that unlocks the premium feature. When new users download the app for $10 they just get the basic features. Since another "version" of the app is available the new users can just buy the premium features if they so choose.
Fast forward to version 5 of the app. Now there's 4 upgrades available via in-app-purchase. However users are only prompted to purchase the next version based on their in-app-purchase history.
I don't think there's a problem for new users who purchase the updated app.