From the very first day of availability, Microsoft has been so determined to get Windows users to upgrade to Windows 10 that it offered free upgrades. Anyone who was rocking a copy of Windows 7 or Windows 8/8.1 had an entire year to upgrade to Windows 10 at no cost, and that offer still stands for anyone who uses the accessibility features

Source: Hot Hardware – Microsoft Entices Windows 10 Enterprise Subscribers With Free Upgrades