If you've installed Office lately, you've probably noticed that by default, the 32-bit apps installed, despite the fact that this just seems antiquated in the modern world. 64-bit options have been available for years, but Microsoft has left the 32-bit apps as the recommended option.
Today at Ignite 2018, the firm announced that moving forward, 64-bit will be the new default. This will be true for both Office 365 and the new perpetual Office product that was released today, Office 2019.
This only goes for new installations though. You're not going to automatically be upgraded to the 64-bit apps. In fact, there isn't a path to go from 32-bit to 64-bit at all, unless you're upgrading a perpetual license to Office 365 or Office 2019. In other words, it has to be a full product installation.
This, of course, applies to Windows PCs, as all Macs have been 64-bit for years, and Apple has been warning users of using 32-bit apps in its latest updates. Indeed, 64-bit Office on Macs has been the standard for some time, and Windows is finally catching up to that.