Techarp: "How Does Windows Anytime Upgrade Work?
Windows Anytime Upgrade (WAU) gives users the ability to upgrade to a higher edition of Windows 7 in minutes. To initiate the upgrade process, an end user will purchase a software license key from a retailer or directly from Microsoft through the online process in the Windows Anytime Upgrade feature, enter it when prompted, and press the "upgrade" button. Before upgrading, the end user need only save any open files and close all open programs. Programs, files, and settings on the machine will remain unchanged."