Users are free to upgrade, of course, but there's a small caveat.
Privacy advocates will undoubtedly find that tidbit to be a bit of a bummer, though it's not necessarily the deal killer that it may appear to be. You can opt out of receiving interest-based and contextual-based advertising that results from having your messages and communications content analyzed by changing the appropriate setting in the Ad Interest Manager. It's unclear, however, if Yahoo will continue to scan your messages even if you opt out of receiving personalized ads.
Don't like the change? Yahoo says you have two options: Switch to IMAP and use a third-party email program like Outlook or Thunderbird, or close your account.