A number of people are asking the same question with similar concerns.
There are several options available for you.
#1) These link are still working today, July 16, 2011 and will RETURN and launch your email back to Classic Mail.
as will this link: http://us.mc598.mail.yahoo.com/mc/welcom…
#2 I have a Yahoo! Plus account, there is an option there to switch back to Yahoo! Classic, which I have done and it remains, and perhaps that is an option for you. The Yahoo! Plus account is $19.99 (US) a year.
#3 Numerous solutions for various browsers and different versions of Windows people use, this is the best solution. Link to: is http://answers.yahoo.com/question/index?…
The link is to an answer given by another person, Katie M, who posted a very good answer that addresses browsers and Windows here on Yahoo!Answers to help you switch back.
In all cases let Yahoo! know that you would like the Classic back.
Hope that helps.