This plugin finds strings in your database and replace the string. You can search in ID, post-content, GUID, titel, excerpt, meta-data, comments, comment-author, comment-e-mail, comment-url, tags/categories and categories-description. It is possible to replace the user-ID in all tables and the user-login for more security in the WordPress-Application.
The plugin homepage is in German: http://bueltge.de/wp-suchen-und-ersetzen-de-plugin/114/
The WordPress admin side of the plugin is in English.