title: How to persist data after Windows Phone 8.1 App update from the store?
body: <p>I will have a pre-installed App in a device with version 2.0
  If the user updates the App to version 2.1 by downloading it from the Windows store,  i want to still know that the user originally had the pre-installed app. (it is a special kind of user which had the app installed when he bought the phone).</p>
  
  <p>How can i persist data between updates?
  Does isolated storage work for this case?</p>