Permalinks Migration is a plugin for WordPress that allows you to safely change your permalink structure without breaking the old links to your website and even without affecting your search engine rankings.
It does this by generating a “301 Redirect” or a Permanent Redirect when a user or spider visits your site through old permalinks and redirects them to the new permalinks of the same post.
I found this plugin via Everton’s blog who recently changed his permalinks using the Permalinks Migration plugin from something like this
www.connectedinternet.co.uk/2005/12/05/652/
to a better Permalink Structure like this
http://www.connectedinternet.co.uk/2005/12/05/post-title/
A good permalinks structure will help you improve your search engine rankings, so if you currently use a bad permalink structure like
http://www.example.com/?p=57
you can safely change your permalink structure to a better one using Permalinks Migration without fear of losing your traffic and pagerank 