WetGeek Even if it no longer has the same disk in it that Windows was installed on? I assumed that the original product key would have been secreted away on that disk. Would it actually have been burned to the BIOS instead?
Windows authenticates against a Microsoft database which records OEM Windows licensing using the motherboard's unique ID. The OEM license, in other words, follows the motherboard. Once licensed, always licensed.
Retail licenses (for example, bought for a home-built), on the other hand, follow the installation disk and you have to have the 25-letter license code.
I updated six 2014-era Dell Optiplex computers at the railroad a few years ago, installing clean using the Media Creation Tool. The computers had Windows 8 OEM installed. Microsoft recognized the computers as Windows-OEM-licensed and authenticated Windows 10 without a burp.
If you decide to go that route, ask, and I'll do a step-by-step for you.