title: Rails: From UUID to ID
body: <p>I've created a model <code>Order</code> with UUID as the primary key.
  Now, I'd like to change it back to a normal incremental integer. </p>
  
  <p>I've tried to run a migration like:</p>
  
  <pre><code>add_column :orders, :id, :primary_key
  </code></pre>
  
  <p>But it obviously doesn't work because there is already an <code>id</code> column.</p>
  
  <p>I've found many posts on how to go from id to uuid, but not how to reverse it.</p>