With an eye on its new servicing schedule, Microsoft will make Update 1 for Windows 8.1—which was initially released back in April—a mandatory update starting this patch Tuesday, August 12. What this means is that Windows 8.1-based PCs will need to have Update 1 installed before they will receive any further updates after Tuesday.
If that seems a little aggressive to you, consider this: The software giant originally required all users—consumers and businesses—running Windows 8.1 to have Update 1 installed by May 12 (that month's Patch Tuesday, and just one month after Update 1 was originally released). But a glitch in the update process triggered a lengthier grace period.
Under the new scheme, consumers had to be updated to Update 1 by June 10 (yes, that month's Patch Tuesday). Businesses were given until August 12, which is suddenly upon us.
To be clear, this requirement applies only to Windows 8.1 (and Windows RT 8.1): If you're still on Windows 8 (like 8.0, including Windows RT 8.0), you are not required to upgrade to Windows 8.1, let alone Windows 8.1 Update 1. (Though, frankly, you should upgrade: Both Windows 8.1 and Update 1 include major enhancements over base Windows 8.0.) Windows 8.0/RT 8.0 is fully supported through January 10, 2023, with mainstream support ending in January 2018.
So why require Windows 8.1 to be so aggressively updated? Windows 8.1 was the first desktop OS Microsoft created under its new serviceability scheme, and in order to ensure that customers are up-to-date with the latest fixes, it needs to move the platform forward more frequently than it did in the past. The Windows 8.1 update schedule more closely aligns with that for online services like Office 365 than it does for traditional Windows and Office versions.
In the past, the security baseline—the level to which you needed to be updated in order to get future updates—was always a service pack. But since Microsoft isn't really making service packs any more ("per se," it is in fact still releasing service packs for some products), the new baseline is some midstream Update (capital U), or what's still called a General Distribution Release (GDR) internally. If it helps you wrap your mind around it, think about it like this: Update 1 is really just Windows 8.1 Service Pack 1 (SP1), and now these updates are being delivered (and required, for future updates) far more frequently than in the past.
Here's an even simpler way to look at it: Update 1 is the baseline for Windows 8.1 support going forward.