OnSong Premium is completely optional and if you've paid for OnSong Pro, then you will always keep all the features you've come to know and love. OnSong is simply transitioning into a subscription app while honoring our valued users. This is done by creating a special OnSong Pro tier exclusive to our current users and available for purchase to new users. No matter what, OnSong Pro users will always belong in that tier and have access to all the functions of OnSong, and we will continue to improve OnSong Pro with bug fixes and updates just as we always have.
If you've paid for in-app purchases, you'll also retain access to all of those without a subscription.
If you have any questions about OnSong Premium, please visit http://onsongapp.com/premium for full details.