When you purchased OnSong originally, the purchase was made with your Apple ID. This is an account that is controlled by Apple and through which you make all your purchases on the App Store. However, none of your information is shared with OnSong. For years, OnSong has offered an optional registration step so that we can keep in touch with you about important news about your purchase.
In December 2019, we've started an account-based system to allow us to provide additional services that can be shared across multiple devices and apps. Since the account system is new, you would just need to create an OnSong account so that we can continue to provide service through the app.
In November 2020, we released a free version of OnSong on the App Store and changed your original purchased to OnSong Pro to reflect the features that you have access to from your original purchase. In OnSong 2020.7, we've completed the transition to this new model for our OnSong Pro users, but this change also is prompting our Pro user to sign in. You can simply close this prompt, however we are making a change in the next update to not prompt you, but you can prevent this sign-in screen by creating an account. Afterwards, choosing a subscription membership is completely optional.