This article explains that user data is unaffected when you choose to use the newest version of Yahoo! Mail.

When you upgrade to Yahoo! Mail, nothing changes in your account or in the information stored in Yahoo! Mail. All emails, attachments, and other content stay in place, and you can access them as you always have, by signing in to your Yahoo! Mail account.

The differences you'll notice in the new Yahoo! Mail are in how the Inbox and other parts of the user interface appear, but the actual contents of your mailbox will be completely unchanged. For a preview of the appearance of the new Yahoo! Mail, visit this web page.