***UPDATED 10/25***
It seems that one of the more frequent questions popping up in the comments on this blog has to do with switching from original (or “Classic”) mail to the to the New mail, or vice versa. I’ve taken a stab at answering it a few times within comments, but maybe a full fledged post will help get some more eyes on the information.
Can I switch between old and new Yahoo! Mail?

For sure! We understand that it’s not easy to switch away from a format that you’re really comfortable with. So 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.

Under where it says “Welcome, yourname” at the top of your Yahoo! Mail page, there’s a “Mail Classic” link (“Switch Back” for those still in Beta). Click that link to go back to the Classic, or Original, Yahoo! Mail version you’re already familiar with.
There is also an opportunity to switch if you visit the Options drop-down menu. At the bottom of the menu is the option to “Switch to Yahoo! Mail Classic…”
After initiating the switch, from either link, you will see a pop-up informing you that you are about to switch back, asking you to confirm or cancel, and asking for feedback. If you do decide to switch back this is one of the best opportunities to be heard regarding why.

To come back 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 ” The new Yahoo! Mail has arrived! Try it now!” appears in the upper-right corner, just below the “Options” link. Click “Try it Now!” to get back to the new Yahoo! Mail.

So now you have no excuse not to try the new version. 
Hopefully this helps clear things up.
***UPDATE***
I’ve heard that some people may not be seeing the same link options to upgrade back to New Mail once switching back to Classic. You should still see the Purple Box shown above if you click on the Options drop-down and select “Mail Options”, but you should also now see also see a link in the very top right of the window (a little to the right of your Yahoo! ID) that reads “All-New Mail”. Clicking there will also take you to the New Yahoo! Mail.
Hope that helps.

Ryan
Community Manager
Yahoo! Mail