title: Migrating data from "Many-To-Many" to "Many-To-Many Through" in django
body: <p>I've got a model</p>
  
  <pre><code>class Category(models.Model):
      title           = models.CharField(...)
      entry           = models.ManyToManyField(Entry,null=True,blank=True,
                                               related_name='category_entries',
                                               )
  </code></pre>
  
  <p>That I wish to refactor to have additional data with each relationship:</p>
  
  <pre><code>class Category(models.Model):
      title           = models.CharField(...)
      entry           = models.ManyToManyField(Entry,null=True,blank=True,
                                               related_name='category_entries',
                                               through='CategoryEntry',
                                               )
  </code></pre>
  
  <p>But south deletes the existing table.  How can I preserve the existing m-t-m relationships?</p>