title: Change column type with django migrations
body: <p>Consider this structure:</p>
  
  <pre><code>some_table(id: small int)
  </code></pre>
  
  <p>and I want change it to this:</p>
  
  <pre><code>some_table(id: string)
  </code></pre>
  
  <p>Now I do this with three migrations:</p>
  
  <ol>
  <li>Create a new column <code>_id</code> with type string</li>
  <li>(datamigration) Copy data from <code>id</code> to <code>_id</code> with string conversion</li>
  <li>Remove id and rename <code>_id</code> to <code>id</code></li>
  </ol>
  
  <p>Is there a way to do this with only one migration?</p>