title: UIManagedDocument migrate data model
body: <p>I am working on an iPhone app that uses a subclass of <code>UIManagedDocument</code> and stores its documents on iCloud.</p>
  
  <p>It was all working fine until I changed my core data model / scheme (adding a new model version - like I had several times in the past few weeks). 
  I added a new property and changed the data type of one of the existing properties.</p>
  
  <p>Now when I run my app I don't seem to be able to load my documents with <code>UIManagedDocument</code>'s <code>-openWithCompletionHandler:</code>. 
  I can create new documents and read/write those.
  If I change the data model version back 1 then I am able to read the existing docs, but not the new ones.</p>
  
  <p>From what I understand I am only do lightweight migrations to the data model and <code>UIManagedDocument</code> is supposed to handle that right?</p>
  
  <p>Any advice would be greatly appreciated!</p>