Applies existing support policy to Windows 8: Customers must upgrade within 24 months to continue receiving patches and fixes.
Computerworld - Microsoft today said that Windows 8.1, slated for release this fall, will use the same lifecycle support timeline as 2012's Windows 8, meaning that it will be supported until early 2023.
Windows 8 users will also be required to upgrade to 8.1, and presumably in the future to newer versions of the OS, to continue to receive security patches and other bug fixes, just as they have been obligated to keep up with past editions of Windows.
"The lifecycle of Windows 8.1 will remain under the same lifecycle policy as Windows 8 with support ending 1/10/2023," said Erwin Visser, who heads Windows marketing to businesses, in a Tuesday blog.
Microsoft's current support life-cycle for Windows 8 pegs the end of "Mainstream" support on Jan. 9, 2018, and "Extended" support's end on Jan. 10, 2023. Under mainstream support, Microsoft patches security vulnerabilities and provides non-security bug fixes. Extended support is limited to security-only updates.
For the first time, the company also confirmed that it will manage Windows 8 support the same way it has previous editions of the operating system.
"Windows 8 customers will have two years to move to Windows 8.1 after the General Availability of the Windows 8.1 update to continue to remain supported under Windows 8 life-cycle," Visser said.
"That's key," said Rob Helm, an analyst with Directions on Microsoft, ranking the importance of the clarification to enterprises.
With previous editions, Microsoft cut off support for the initial build of Windows, known as RTM for "release to manufacturing," two years after the release of the first service pack. If it shipped a second service pack, it stopped supporting the first 24 months after the follow-up's debut. Service packs have historically been free, just as Windows 8.1 will be.
But Microsoft has done away with service packs for Windows 8, instead adopting a faster-paced development and release schedule that will ship a new version about once a year.
Helm had expected that Microsoft would make Windows 8.1 mandatory, just as the firm demanded customers upgrade from Windows 7 RTM to SP1.
"Windows 8 has a relatively small base at the moment [and] Windows 8.1 is free, so it's not insanely onerous to require users [to upgrade]," said Helm.
If Microsoft had changed its life-cycle policy for Windows 8.1, it would have been forced to maintain an increasing number of versions. "This makes sense because under its faster-release schedule, if it didn't pull up the carpet, there would be too many different versions to support," Helm said.
Visser also announced the availability of a preview of Windows 8.1 Enterprise -- the version designed for Microsoft's volume license customers -- in the same blog today.
"In general, Windows 8.1 is a much better target for migration than Windows 8," said Helm, echoing other analysts who have said the same. "We've been recommending Windows 8.1 as the alternative to Windows 7 to clients getting off Windows XP."
Helm cited Windows 8.1's new enterprise-oriented features for that recommendation, including "workplace join," which will allow trusted devices to access secured data on a company's network, and mobile management additions such as limiting access to a single "Modern," née "Metro," app.
"The renewed focus on mobile management will make it much easier for enterprises to handle BYOD," Helm added, referring to the "bring your own device" trend where businesses support workers' choice of hardware.
Also a big plus in Helm's book was Microsoft's renouncing its stubborn insistence that it knew what customers wanted. "Just the overall fact that Microsoft took the time to understand what enterprises needed, and delivered on those features, which we really could have liked to see in the initial release," has been encouraging, said Helm.
The enforced upgrade to Windows 8.1 is the strongest signal yet that Microsoft will offer future versions in the Windows 8 line -- Windows 8.2, for instance, or even 8.3 -- free of charge, just as it has service packs in the past. Deviating from free without also eliminating the upgrade requirement for continued support would likely trigger a revolt by customers, or at least a hue-and-cry that Microsoft would find hard to silence.
When Microsoft first announced it would speed up Windows releases, many experts assumed that the company would dispense free Windows 8 upgrades for several years running before launching an entirely new edition, perhaps titled Windows 9, that would come with a price tag.
Microsoft has not yet set a ship date for Windows 8.1, or as Visser put it, the version's General Availability. When it does, the 24-month clock will start ticking: An October launch of Windows 8.1, for example, means customers will have until October 2015 to finish their upgrades.
Windows 8.1 Enterprise Preview can be downloaded from Microsoft's website. The sneak peak will expire Jan. 14, 2014.
This article, Microsoft mandates Windows 8.1 upgrade, was originally published at Computerworld.com.
Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at 
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@gkeizer, on Google+ or subscribe to Gregg's RSS feed . His email address is gkeizer@computerworld.com.