@Xedur, if I understood it right, ALL users who downloaded the app before will have access to the new paid version, but what Odisej want’s is only the ones who in-app purchase in the free version not all.
When you move an app to paid, all users who would got it will have the paid version for free. So you will need to create a new version (new SKU) if you don’t want this.
Creating a new app you can give promo codes to the ones who already bought the last version. Maybe you can edit the old version and introduce an option to create a promo code so you can use it to buy for free the new one. please note that promo codes are very limited so I don’t know the number of users already bought your app.
You can also limit the current app to a almost 0 function without paying it, promoting to uninstall it for free customers. When you have almost all only paid costumers you can pass the free version to paid version. Even the free persons will benifit the new version but they are a few.
I bet someone will give a much better answer than mine, since I’ve 0 experience on this field.