There are two versions of OnSong available on the App Store, OnSong Pro and OnSong. If you had purchased OnSong from the App Store in the last 8+ years, it has since been renamed OnSong Pro. That's because we have added lots of features to the original app over the years, bringing your experience into the "Pro" tier. However we've found that OnSong has become so feature-rich that some users just need the basics, and there are many more musicians that just need to receive and view OnSong songs and sets. That's why we've made OnSong a free app on the App Store.
If you've purchased a new iPad or iPhone, you may be excited to download OnSong on that device and start using it. Likewise, if you had deleted OnSong and want to reinstall it, or have changed Apple IDs, you may have gone into the App Store to download OnSong. You've likely been presented with two choices and have downloaded the free version. This version requires you to create an account for use, but after creating an account, prompts you to choose a membership level. While you can use this version for free without a membership, it is limited. You can't restored libraries and you're limited to 30 songs. In addition, you can't access many of the great features you've come to know and love. At this point, you may return to the App Store and download OnSong Pro. You may then find yourself signed in with the free account on the Pro version which restricts your features.
In order to use OnSong Pro and the features it comes with, you'll just need to sign out of the free account. To do this, go into the Utilities menu by tapping on the gear icon in the Menubar and then choose the Account option to view the Account Screen. Scroll to the bottom of the screen and tap on the Sign Out button. This will then revert OnSong to use the "base" plan of the app which is the Pro tier. This gives you access to the OnSong features you're used to using.
In the future, remember that apps are purchased with your Apple ID and can be downloaded and installed at any time. Just open the App Store app on your device and access your Account to view your Purchases. You can then download and install OnSong Pro for free on any device where you use the same Apple ID as the original purchase.