I'm guessing email. You can flip back and forth from Classic:
"For sure! We made it super-easy to switch back whenever you want. You can switch up to the new and back to the old as often as you want. We hope you'll see how much the new Yahoo! Mail improves on the old version. But until you agree, here's how to switch back.
To go to the new Yahoo! Mail, just click "Options" in the upper-right corner of your Yahoo! Mail page. On the Options page that opens, the message " Yahoo! Mail Beta has arrived! Try it now!" appears in the upper-right corner, just below the "Options" link. Click "Try it Now!" to get to the new Yahoo! Mail.
To come back to the old Yahoo! Mail, under where it says "Welcome, yourname" at the top of your Yahoo! Mail page, there's a "Switch Back" link. Click "Switch Back" to go back to the Yahoo! Mail version you're already familiar with."