The W10 deployment is a comedic trailblazer of retarded shit. You wanna know what my experience was?
a. Okay, let's upgrade to Windows 10! You get a free upgrade, right? So how do I claim my Windows 10 key? Err... dunno. The "Get Windows 10" thing doesn't seem to exist. I assumed (correctly) that it was because I wasn't using an 'eligible' key, given I'm on W8.1 Enterprise. So I just download the Media Creation Tool and just go at it. I guessed at the time that they just added the means to upgrade with the key you have within W10 itself.
b. Media Creation Tool is a fucking nightmare. It is a barebones direct download tool that apparently uses Windows Update stuff, Idk. It doesn't keep track of progress. It doesn't let you pause. It doesn't let you do anything to the download process. And it doesn't tell you what it's doing at all. I set it to download as an ISO to my 3TB drive. 2 hours later, I discover it is still at 0%. Literally no progress. Is it downloading? Is it not? I don't know.
c. Media Creation Tool does not exit cleanly. I had to force exit it a bunch of times. I had to update Windows 8.1 with a total of 1.5 GB updates, and clean out a shitton of space on my drive to get the tool to work. I won't even go into detail of how long the actual download took.
d. Finally made a bootable usb with the downloaded iso. Replaced my Corsair Force drive with the Samsung 840. Unplugged 3TB drive to not have any masterboot or whatever bullshit. Booted computer. Got to installing W10. Fast-forward to me watching screen go "Hi, we're doing stuff". Go through their menus. More menus. Somehow wind up with "Hi we're doing stuff" again. Uh, okay.
e. Finally in Windows 10. Let's valida-- nope, it has no means to upgrade my W7 Ult key to W10 Home. Did I miss something? Did the offer expire? What about the store, is there something there I need to 'purchase' to do this with? Is there a program? Nope, nada, nothing.
f. After a shitton of troubleshooting I finally get a hold of a Microsoft rep. Took its sweet time to get through. What's he say? That I need to downgrade to Windows 7, validate my key there, then upgrade to W10.
TL;DR - I had to downgrade, activate then upgrade. And if I was to ever lose my key I would have to redo this process again.