Microsoft has just announced the first pricing information for Windows 10 at its preview event today. The biggest news is that the new OS will be completely free for current Windows 8.1 and Windows 7 users for its first year of availability—after that time period has expired, OS upgrades will presumably need to be paid for as they are currently (though Microsoft was less than clear on this point, it made no mention of a paid, Office 365-style subscription for Windows upgrades). The Windows 10 upgrade for Windows Phone 8.1 users will also be free.

"Once a device is upgraded to Windows 10, we'll be keeping it current for the supported lifetime of the device," said Terry Myerson, executive vice president of the Operating Systems Group. "With Windows 10, we think of Windows as a service... The question 'what version are you running' will cease to make sense."

Windows 7 and Windows 8.x collectively run on well over half of the world's Windows PCs, meaning that a wide swath of existing Windows users will be able to make the jump to Windows 10 free of charge. A larger user base, especially in the first year after Windows 10's official release, increases the chances that developers will target Windows 10 and its new APIs.

Users of the Windows 10 Technical Preview have already gotten a taste of this continuous stream of updates, and the release version of Windows 10 will be no different. As we've covered previously, Microsoft will offer different "channels" for different users—one that gets new features as soon as they're available, one that never gets new features, and one that allows administrators to roll out new features at their own pace. Security updates will be published at the same rate they are today. This new update scheme is an effort to balance delivering new features to consumers while not upsetting businesses that prefer to roll updates out more slowly.

Apple has offered the last two versions of its OS X desktop operating systems for free, but this is a first for Microsoft—it has offered discounts for new Windows upgrades in the past, but it has never made them available for free.

Update: Microsoft fielded some questions about this upgrade in its Q&A session after the event. The company "hasn't decided" how it will handle upgrades from Windows 7 or 8.1 after the first year of Windows 10 availability ends, and it is "working on an update for Windows RT," but doesn't have further details to share.

Update 2: A blog post from Terry Myerson clears up what "Windows as a service" means, though the duration of "the supported lifetime of the device" is still foggy. "This is more than a one-time upgrade," writes Myerson. "Once a Windows device is upgraded to Windows 10, we will continue to keep it current for the supported lifetime of the device—at no additional charge."