As noted below, from windows.com:
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.
Emphasis added by me.