Site Address Migrator is a WordPress plugin by sourcefound that migrates urls when you change the Site Address:
Updates matching links in the description, content and excerpts of all posts, pages and media (including custom types)
Updates matching links in post and page option fields (supports serialized data)
Updates matching links in the content of all comments
Updates matching links in post category and tag descriptions
Updates matching links in user descriptions
Updates matching links in user website fields
Updates matching links in WordPress and widget option fields (supports serialized data)
To bypass any of the above, simply comment out the appropriate line in the PHP file (that calls the sf_mgr_update_table function).
Backup your MySQL database before making major changes like your Site Address. This plugin has not been tested for Multi-Site installs, use at your own risk!
Download: http://wordpress.org/extend/plugins/site-address-migrator/