Drupal7: How to change an already existing text field to a text area

Here is a useful snippet that you can use to change an already existing drupal textfield into a text area. Just replace the field_machine_name with the machine name of the field that you would like to change.

 'varchar',
    'length' => '20000',
  ));
  db_change_field('field_revision_field_machine_name', 'field_machine_name_value', 'field_machine_name_value', array(
    'type' => 'varchar',
    'length' => '20000',
  ));
  // Clear caches.
  field_cache_clear(TRUE);
?>