Microsoft ClickHere say: "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"
Near the bottom of page the small print says:
"It is our intention that most of these devices will qualify, but some hardware/software requirements apply and feature availability may vary by device. Devices must be connected to the Internet and have Windows Update enabled. ISP fees may apply. Windows 7 SP1 and Windows 8.1 Update required. Some editions are excluded: Windows 7 Enterprise, Windows 8/8.1 Enterprise and Windows RT/RT 8.1. Active Software Assurance customers in volume licensing have the benefit of upgrading to Windows 10 enterprise offerings outside this offer. We will be sharing more information and additional offer and support terms in coming months. "