There is still some confusion over the subject of how this upgrade will work.
It is shaping up to a view that it is not exactly free. You will have to prove, and give up, your genuine Windows 8/7 key, which will then be embedded in the Windows 10 installation.
I know it is a majority situation, that many users have the same key on more than one household computer, but that is another issue. Speaking strictly of the MS Eula, you will only be able to uninstall and return to your earlier OS, if you are unhappy with 10.
So, maybe it can be viewed as getting two for the price of one........................?