Microsoft's Windows 10 site says ...
Microsoft Free Upgrade Offer wrote:
Great news! We will offer a free upgrade to Windows 10 for qualified new or existing Windows 7, Windows 8.1 and Windows Phone 8.1 devices that upgrade in the first year! And even better: once a qualified Windows device is upgraded to Windows 10, we will continue to keep it up to date for the supported lifetime of the device, keeping it more secure, and introducing new features and functionality over time - for no additional charge. Sign up with your email today, and we will send you more information about Windows 10 and the upgrade offer in the coming months.
Windows 10 retail launch is expected to be Q3/2015. So, you should be able to use the Free Upgrade Offer anytime until Q3/2016 - this would "permanently" upgrade your Windows 8.1 to Windows 10 at no cost. In essence, your Win8.1 Product Key would be transferred to Win10 (in Microsoft's evil genuine product registration database). The only possible downside (aside from the possibility of Win10 really sucking) is that you may no longer be able to use your
Win8.1 Win10 Product Key to activate a copy of Win8.1 - no one outside of Microsoft really knows details about this yet, of course.
The Windows 10 beta (Technical Preview) was originally scheduled to terminate 15Apr2015, but it's still active a month later. People report that this Free Upgrade Offer was originally intended to also include Windows Vista, but (if true) this has apparently changed. Users of older Windows versions (XP, etc) are clearly past the cutoff.
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