Drupal update hook to copy a field into another

rs_update_7100.php

/**
 * Implements hook_update_N().
 * 
 * Copies title for legacy_content type.
 */
function rs_update_7100(&$sandbox) {

  if (!isset($sandbox['max'])) {
    $query = db_select('node', 'n');
    $query->addExpression('COUNT(*)', 'count');
    $query->condition('n.type', 'legacy_content');
    $sandbox['max'] = $query->execute()->fetchField();
    $sandbox['current_position'] = 0;
  }

  if ($sandbox['max'] > 0) {
    $limit = 10;
    $nids = db_select('node', 'n')
      ->fields('n', array('nid'))
      ->condition('n.type', 'legacy_content')
      ->orderBy('n.nid')
      ->range($sandbox['current_position'], $limit)
      ->execute()
      ->fetchCol();
    $nodes = node_load_multiple($nids);
    foreach ($nodes as $node) {
      $node->field_title[$node->language][0]['value'] = $node->title;
      node_save($node);
    }

    $sandbox['current_position'] += $limit;
    $sandbox['#finished'] = $sandbox['current_position'] / $sandbox['max'];
  }
  else {
    $sandbox['#finished'] = 1;
  }

  if ($sandbox['#finished'] >= 1) {
    drupal_set_installed_schema_version('rs', 7100);
    return format_plural($sandbox['max'], '1 node updated', '@count nodes updated');
  }
}