The title pretty much says it all, but I can elaborate. I have a paid iOS app that has plenty of existing customers. I think i want to convert to a free app now, and allow full upgrade via in-app-purchase. The problem is, I don't want to make my existing customers buy the app again to use it, nor do I want to make it easy for hackers to just flip a switch and get the pro version.
What is the most secure way to "Grandfather In" existing users of a paid iOS app that will go free?