It seems you can no longer switch back - this is taken from the Yahoo Helpline
Can I switch back to a previous version of Mail?
Last Updated: 06 September 2011
We have removed the "Return to Original Mail" link from the help menu as we're strongly encouraging all users to migrate to the newest version of Yahoo! Mail. We will continue to invest heavily in the latest version of Yahoo! Mail, further improving it and delivering more compelling features to our users.
We know that changing to a new interface can seem daunting, so we’ve done everything possible to make the transition easy. We have created help pages specifically for users who have recently switched to the newest version of Yahoo! Mail, highlighting differences that you may notice: