All you need to do is to run the following MySQL quesry in phpmyadmin:
Replace - with _ permalinks
About Ovidiu Purdea
Ovidiu Purdea, WordPress Webmaster and SEO & SEM specialist with over 9 years experience in this field. Guru using, modifying, adapting websites based on WordPress. You can follow Ovidiu Purdea on my Google+ profile.