http://www.microsoft.com/en-us/windows/features?ocid=win10_auxapp_LearnMore_win10

*Windows Offer Details

Yes, free! This upgrade offer is for a full version of Windows 10, not a trial. 3GB download required; standard data rates apply. To take advantage of this free offer, you must upgrade to Windows 10 within one year of availability. Once you upgrade, you have Windows 10 for free on that device.

Our lawyers made us say this:

Windows 10 Upgrade Offer is valid for qualified Windows 7 and Windows 8.1 devices, including devices you already own. Some hardware/software requirements apply and feature availability may vary by device and market. The availability of Windows 10 upgrade for Windows Phone 8.1 devices may vary by OEM, mobile operator or carrier. Devices must be connected to the internet and have Windows Update enabled. 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 to upgrade to Windows 10 enterprise offerings outside of this offer. To check for compatibility and other important installation information, visit your device manufacturer’s website and the Windows 10 Specifications page.

1 Apps sold separately; availability may vary.

2 Cortana available in select global markets at launch; experience may vary by region and device.

3 Features only available in supported games in Xbox Live-supported countries, see xbox.com/live/countries. Limited number of games available in 2015 support cross-device play; additional games to follow. Stream to one device at a time; Streaming with multiplayer from Xbox One requires home network connection and Xbox Live Gold membership (sold separately); Gold also required for multiplayer play on Xbox One. DirectX 12 only available with supported games and graphics chips.

很明确的说了在发布后合资格的系统一年内免费升级了并且永久免费！！！