In this Ask A Dev, iOS architect Conrad Stoll draws upon the experience he gained from his Runtime iOS app to tell us what to consider when making a free version of a premium app.
According to Conrad, the first thing you need to consider is your code base, and how not to mess it up by disabling features. You'll also need to lean on analytics to know which features are most important to your users.
From there, you have to decide things like whether or not you want include in-app purchases and how to give premium users a better experience without alienating your free users. As Conrad says, "Converting to a free app isn’t difficult, but it does require a lot of thought to do it well."