Windows 10 is changing [the old licensing model]. As announced
last week, current users of Windows 7, 8, and 8.1 will be able to upgrade to Windows 10 for no cost. Microsoft says that this will be a limited time offer that will run one year from the release of Windows 10 [...] Subsequently, and in some ways even more importantly,
those new Windows 10 machines will continue to receive updates "for the lifetime of the device—at no cost." Those paid major version upgrades that Windows has historically had will be going away; once you're on the Windows 10 train, you're on the
Windows train, and you'll continue to receive a steady stream of updates and upgrades until your system stops working and you buy or build a new one.