Yahoo officially stated that:
Those who upgrade can opt out of contextual ads through Yahoo’s Ad Manager. Yahoo bluntly tells users who refuse its new policies that they should either download their mail to another IMAP client, or close their account. Premium Mail Plus users who want to cancel their accounts can get a prorated refund. Basically all free Yahoo email accounts will be forced to be scanned for their target ads. Looks like what Yahoo really eliminates is not the Classic Mail, but the free of use of their email service. Yahoo is following Google. Gmail has long scanned your email to show you related ads.