title: When performing a Core Data migration, how can I carry over old values from the source store from a different entity?
body: <p>I'm performing a migration using Core Data's standard mapping model approach, and it seems to be working fine, except for one thing: I've moved an attribute from one entity to another, and I want to move the underlying value from the source store to that new entity as part of the migration.</p>
  
  <p>This <em>should</em> be possible, but I'm having trouble figuring out which steps I need to take to accomplish this.</p>