title: Convert data on AlterField django migration
body: <p>I have a production database and need to keep safe the data. I want to change a Field in model and convert all data inside that database with this change.</p>
  
  <p>Old field</p>
  
  <pre><code>class MyModel(models.Model):
      field_name = models.TimeField()
  </code></pre>
  
  <p>Changed field</p>
  
  <pre><code>class MyModel(models.Model):
      field_name = models.PositiveIntegerField()
  </code></pre>
  
  <p>Basically I want to convert the TimeField value (that has a Time object) in minutes.</p>
  
  <p>Example: I have in an object <code>time(hour=2, minute=0, second=0)</code> and I want to convert that field value in all database table to <code>120</code> when I apply the migrate.</p>