Windows 10 didn't want to activate as a hardware refresh from the old system. MS support kept telling me I needed the old Win7 Ultimate key from over 10 years ago. When they tried to escalate, I got transferred to a number that was closed for the weekend. So I decided to try my luck with a grey market key. It was bad luck. Fortunately I spent the extra $5 for buyer protection, and filed a ticket to see if I can get a refund without going through a paypal dispute.
Tried clearing all keys and trying again with MS support. Nope. Kept repeating that I needed the old Win7 key even though I told him repeatedly that I didn't. Then I realized I had that surplus Optiplex. I was certain that key had never been used to upgrade. Dude now said if I wanted to do that, I'd have to install Win7 on the new box first. NO. That wasn't going to happen. Since I was getting nowhere with MS, I said a curt goodbye and hung up. To the Google I went.
Turns out activating a Win10 install with a Win7 key IS a thing. A few moments of uncertainty later, and I have an activated copy of Win10 with a legit key. Take THAT, phone support guy!