title: Django Rename and change data type of model field live with zero downtime?
body: <p>I have a django model in use on a production application and I need to change the name and data type of the field with zero downtime to the site.  So here is what I was planning:</p>
  
  <p>1) Create the new field in the database that will replace the original field<br/>
  2) Everytime an instance of the Model is loaded, convert the data form the original field and store it into the new field, then save the object (only save object if new field is empty)<br/>
  3) Over time the original field can be removed once every object has a non-blank new field<br/></p>
  
  <p>What method can I attach too for the 2nd step?</p>