I'm getting mildly worried. Thought I had read confirmation that all Win10 upgraders would end up with a key which they could use to create a fresh install. (on the computer which requested the upgrade, not any old computer)
As I understand it, if you install over Win 7/Win 8.1, the Win 10 upgrade relies on the resident OS' product key to be authenticated. Once installed a new product ID [found in Control Panel>System... scroll to bottom of page] is generated and can be used for future installations on the same machine.
To clean install Win 10 from a DVD or USB stick, you need a legitimate Win 7 or Win 8.1 product key, which is requested before the installation process begins. Again, once installed a new product ID is generated and is used for updates and upgrades during the lifetime of that machine.
Alternatively, you can download the Media Creation Tool and upgrade directly from the desktop of your current OS. When the download completes an ISO image is created and appears as an icon on the desktop... clicking on this opens a dialog box from which you can choose to install immediately on that machine or to create a DVD or USB.
I'm not going to sweat that too much though, yet. Probably not going to install on any of my PCs for at least another 6 months or so.
I don't plan to upgrade on any of my primary machines, but I have prepared a standalone machine for the purpose of installing and evaluating Win 10. If I like it enough and I'm satisfied the 'phone home' features can be adequately disabled, well maybe then I'll consider installing it on other machines.
For now, though, I am more than happy with Win 8.1 and have it set up to my preferences, so I don't know that I want to mess those machines before I'm convinced that Win 10 is a better proposition.... and for now I'm not. Given that many commenters [like Martin Brinkman] reckon it is still 'half-baked, and that many users are unhappy about various aspects, I think it will be some time before I'm ready to upgrade over what works well for me.