The plugin automatically redirects users who have accessed a blog post link without the category to the one which has the category and therefore avoiding page not found (404) errors.
If you have an existing website and you want to add category to your blog posts links (e.g. by using: /%category%/%postname%/ in Settings > Permalinks) then your visitors
will most likely run into page not found errors. This plugin checks for such post links and redirects the visitor using permanent (301) redirect to the correct /Category/Permalink link IF the permalinks are setup.
This plugin assumes that you have previously used /%postname%/ in your Settings > Permalinks.
Demo
Download it from WordPress plugin directory
http://wordpress.org/extend/plugins/permalinks-to-categorypermalinks/