It IS possible to revert to the previous version of Yahoo Mail. Here's how....
1) Right-click on the desktop.
2) Click on Screen Resolution
3) Reduce the Resolution to 800x600 and press OK. Keep changes.
4) Sign into Yahoo Mail.
5) After a few seconds, a message will come up, saying there is a problem with the resolution.
6) Scroll down and you will find an option to change back to the previous version of YahooMail
7) Click the option and Hey Presto!
8) Revert your screen resolution back to it's highest settings and you're done.