<?php
ini_set('max_execution_time', 0);
$nodes = node_load_multiple(array(), array('type' => 'article'));

foreach ($nodes as $node){
  $currnode = node_load($node->nid);
  $body_html = ($currnode->body['und'][0]['value']);
  $body_html =  str_replace('/sites/mysite/','/sites/default/',$body_html);
  $currnode->body['und'][0]['value'] = $body_html;
  node_save($currnode);
?>