This was/is prompted by something WR posted in the comments on the GOTD Games page this weekend. His son screwed up the BIOS for his PC attempting to upgrade it -- happens. [If you buy a mature model motherboard or PC or laptop etc., meaning older so it's on sale, a rule of thumb is update the BIOS then, while you can return it if flashing the firmware bricks the device. Then if you can avoid it, don't fool with updating the BIOS once the return period's up.] What started it was his son "failed to make a note of his windows activation code", asked friends for help etc. I thought it might be useful to note some things I've found about Win10's activation, codes etc., that's not been published AFAIK.
Win10 activation has changed since it first came out. Nowadays it seems Microsoft stores the ID for the device, using a secret recipe that gathers various IDs & serial numbers from the hardware. Once that device is recorded having a valid Win10 license, you should never have to enter a key again, even if you do a fresh install. If you change some hardware components, e.g. a hard drive, it usually doesn't matter -- Microsoft keeps a running tally of all hardware changes, and only deactivates Win10 once some threshold is met. I've found it handy for me to add a 2nd copy of Win10 to our PCs, & because a copy's registered to that device, it auto activates. It's a hassle booting to a USB stick or drive, so I use that 2nd copy, with almost no software installed, the same way you'd use a bootable USB stick. [Note 2: that's why I use Win10 Enterprise for my Windows To Go drives -- that version will only try to activate from a corp. server, so running it cannot count towards the number of changes Microsoft allows.]
Now, the following is in the category of try it & see if it works... Windows activation has always been a little bit iffy -- for every 10 people that report something works, even normal registration, it seems there's at least one person for whom it did not work.
You can sign into your Microsoft account, where you manage Windows PCs using that same email-based account, and you can deactivate the license for one device, and that *May* allow you to reuse that key on new hardware. You can also try to do that through Settings -> Update & Security -> Activation. If you enter a key & it doesn't work, or it Win10 deactivates, there's an option on the same page to try to repair that activation. You can also call Microsoft, same as in the old days, but finding a number means drilling through their support pages or Googling.
There are reports that using a Win7 or Win8 key still works, though you might have to try it during installation &/or afterward. There are apps, e.g. Nirsoft's prodokey, that report the key that's stored in Windows. In Win10, the key that will be reported Will Not in my experience match the key that you entered. I have no idea if the new key or the old one you originally used will work -- generally the only reason to try either is if you have a new motherboard, and while a license you paid for *might* allow that, the upgrade giveaway licenses do not.