Windows 10 Reinstall After Free Offer Expires

I have a question, which I've searched out online, but have come up with conflicting information. If I have a machine that was upgraded from Win 7, 8, or 8.1 to Win 10 during the free offer period, and need to reformat now that the offer is expired, do I need to purchase Win 10 in order to get it back on? The product key either on the COA or within the BIOS will be for the native OS, and that's obviously what I will have to reformat to at first,but what will happen after that? If I grab the product key from a command line prior to format, will it be for the original OS or for Win 10? And if the computer won't boot, is that it? Do I have to go back to the original OS? I'm seeing so much different stuff online, and I don't want to just give it a shot to see, in case I lose 10. Has anybody had to deal with this yet? Thanks.

 

It's easy but does take some time.

http://forums.majorgeeks.com/index.php?threads/win-10-killing-two-birds-with-one-stone.307942/

 

Thanks for the link, great info. What about systems that won't boot though? After creating this thread last night, I tried something out. I took a laptop I had which originally came with Win 7, and is now upgraded to Win 10, and attempted to grab the product key via the cmd line to compare against the COA on the back. I used:

wmic path softwarelicensingservice get OA3xOriginalProductKey

And although there was no error, it didn't return a key. It was just blank. So as you'd figure, the product key is most likely the one corresponding to the new 10 installation, but why is it unavailable?

 

As long as the Win 10 was previously activated you can just reinstall using the Media Creation Tool burnt to DVD. It will activate automatically, no product key required.

 

Nice. I'm gonna give it a shot tonight on one of my laptops just to check out how it goes. Thanks.

 

It took 6 hours on my wife's laptop, but it will include the anniversary update which accounts for a significant chunk of it.