Via Forbes -
Microsoft Changes Windows 10 'Free Upgrade' Rules
Unclear at this time if it invalidates the key for the original O/S it came from.“If you install this build of the Windows 10 Insider Preview on a PC and it doesn’t automatically activate, you can enter the product key from Windows 7, Windows 8 or Windows 8.1 used to activate the prior Windows version on the same device to activate Windows 10 by going to Settings > Update & security > Activation and selecting Change Product Key,” he explained.
Well, at least it makes the process easier