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.

There is a way to tell the search engines (and browsers) that the page has permenantly moved, and that the old address should be replaced by the new one. It’s called a “301 Redirect”, also known as a Permanent Redirect. When you do this, Search engines will update their indexes quickly and you won’t lose your pagerank.you will continue to receive traffic as though nothing had changed. This works for search engines, bookmarks, and links from other sites.

By now, you know how can you change your permalinks without losing the traffic you’re already getting. You can download this Permalink Migration Plugin to achieve this for you.

This plugin will generates a “301 Redirect” when user or spider visit your site through old permalinks,and redirect them to the new permalinks of the same post. Permalinks Migration Plugin allows you safely change your permalink structure without breaking the old links to your website, and even doesn’t affect your search engine rankings.

Pricing: Free
Requirements: -
Source: http://www.deanlee.cn/wordpress/permalinks-migration-plugin/