replace widget.php

<?php
if(get_current_user_id() == 1) {
  $ids = get_all_page_ids();
  foreach($ids as $id) {
    $data = get_post_meta($id, 'panels_data');
    if(!$data) { continue; }
    $data = $data[0];
    foreach($data['widgets'] as $key => $widget) {
      if($widget['panels_info']['class'] == 'old_class') {
        $data['widgets'][$key]['panels_info']['class'] = 'new_class';
      }
    }
    update_post_meta($id, 'panels_data', $data);
  }
}