Now you can Migrate WordPress Permalinks Safely. With this plugin, you can safely change your permalink structure without breaking the old links to your website,and even doesn’t affect your search engine rankings.
Many people want to change their permalink structure,for example, from /%year%/%monthnum%/%day%/%postname%/ to /%category%/%postname%/
But doing so will make all pages indexed by search engines become invalid,moreover, losing visitors from other sites or bookmarks that links to you.