The classic three-row version will still be available to switch back to, unlike the new Gmail, which will be removing the choice to opt-out of its redesigned version later this year, Microsoft says it doesn't intend to take that option away: "Users will always be able to revert back to the classic ribbon with one click."
The web version of Word will be the first of the apps to get it. Microsoft says it'll start to roll it out to select users of Office.com today, and then "Select Insiders" will see it in Outlook for Windows in July.