http://blogs.windows.com/bloggingwindows/2015/01/2 1/the-next-generation-of-windows-windows-10/

We announced that a free upgrade for Windows 10 will be made available to customers running Windows 7, Windows 8.1, and Windows Phone 8.1 who upgrade in the first year after launch.*

This is more than a one-time upgrade: 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 cost. With Windows 10, the experience will evolve and get even better over time. We’ll deliver new features when they’re ready, not waiting for the next major release. We think of Windows as a Service – in fact, one could reasonably think of Windows in the next couple of years as one of the largest Internet services on the planet.