title: How to check value transition in Django (django-admin)?
body: <p>I have a status field which has 3 values: pending, activated
  and rejected. If I am changing the value of status I want to
  have a check that activated cannot be changed to pending. I
  do not want to write stored-procs for this. Can I have the
  previous value in Django before saving?</p>
  
  <p>Means new and old value.</p>