URLs Unfound – Permalinks

At first i was using the permalinks as http://www.99facts.com/postname/

But now i changed to http://www.99facts.com/category/postname/

Now the visitors who come from google are getting http://www.99facts.com/postname/ but the page is not found because i have changed the permalink. Now i have to redirect http://www.99facts.com/postname/ to http://www.99facts.com/category/postname/ .. how can i do it ?