title: Change paid app to free but know if user previously purchased it
body: <p>I am considering changing my paid <code>iOS</code> app to be free, making it <code>ad based</code>, and having an <code>in-app purchase</code> option to remove the ads.</p>
  
  <p>This sounds like a good idea if I was just launching the app, but we have over 30k paid downloads, and I don't want those users to ever see the ads when they update to the new version which is free.</p>
  
  <p><strong>Do I have any options here?</strong></p>