title: Switching from a paid app to a free app with auto-renewing subscription
body: <p>I have an app which costs $5. I'd like to change this so that the app is free and that users must purchase an auto-renewing subscription to use it. I know how to implement the auto-renewing subscription, but the problem is dealing with users who have already bought the app for $5; I'd like to continue letting these users use my app without a subscription. </p>
  
  <p>The rub is that for privacy reasons I can't store any identifying information on my server which link an account for my app to a specific person (not even UIDID). What I <em>can</em> do is maintain a separate database table which links UIDIDs to subscription purchase receipts which will allow me to know if a user has a subscription.</p>
  
  <p>So my question is, how can I identify users who got my app when it cost $5? I know there's a way to restore in-app purchase receipts, but is there a way to to retrieve a receipt for the initial purchase of the $5 app which I could store on my server?</p>
  
  <p>The poor man's solution is just to mark all current UIDIDs (i.e. the UIDIDs of people who have paid $5) in my server as paid, but then they would have to buy a subscription if they ever wanted to use my app from a different device. </p>