(CNN) -- For Yahoo users who prefer old-school e-mailing, your grace period is over.
Monday is the first day that users will be required to upgrade to the latest version of of Yahoo's e-mail service, which according to various assessments is either the second or third most popular on the Web.
This change has been in the works since December, when Yahoo, under the guidance of new CEO Marissa Mayer, announced a major and long-awaited overhaul of e-mail.
"You've told us loud and clear that you want fewer distractions when it comes to e-mail," Mayer said at the time. "You want to quickly login, communicate, and get on with your day. And we've listened."
The upgrades were made available for Web, Windows 8, iPhone/iPod touch and Android. But as is always the case when faced with change, some folks were more comfortable with older versions, including "Yahoo Classic," and were given until Monday to switch.
In a Yahoo Help post the company noted that upgrading requires users to agree to terms including "automated content scanning." That's technology similar to that used by Google with Gmail, which uses search algorithms, not real people, to read through the content of people's messages in order to target relevant advertising -- as well as other features -- to them.
Users may opt out of that feature by changing their settings.
The changes to the latest version of Yahoo Mail include removing some of the boxes that filled the page in the Web version and offering thumbnail images of attachments, presumably to help avoid clicking on scams, spam and other unwanted content.
On Mail for iPhone, users can select multiple messages and swipe to delete, highlight or file them away. The Android app was upgraded to, among other things, put less drain on battery life.
Still don't like the latest version? The Yahoo help post offers options for disgruntled users, including how to download your content and access it through another client and how to delete your account.