Installed PCI LAN card = Windows De-Activated !

I built this machine with Win7 64 Pro years ago. Then installed Windows 10 when it came out free. Been having network issues so just installed a PCI LAN card and turned off motherboard LAN. Suddenly Windows 10 says it needs to be activated! I've changed graphics cards in this machine a few times but this never happened before.

So I tried all the usual- sign in with hotmail account, wouldn't reactivate... try to extract product key using Nirsoft - just gave me a generic that didn't work.... I know I might be able to find the product key if I tore my house apart looking.

So I actually got through to a real person via the activation troubleshooter. And of course he wanted my Windows 7 64 Pro product key. I asked him to just do a remote connection and I saw him do something with command prompt to get the product key but that didn't work. So he did something else but I couldn't catch it. Eventually it appeared that he did get a Windows 7 64 Pro product key. But when he entered it into activation it said something about "that product key is already in use on another system". Then he did something and a another screen about calling a phone number came up. He put in all these fields of numbers and activation worked!

I did have him copy/paste that Windows 7 64 Pro product key on the desktop, but I'm just wondering- did he actually find my original Windows 7 64 Pro product key or did he just pull one out of his hat? If he did get it, I'm curious how ?? Because I've got several more Windows 7 machines here that I upgraded to Win10. I would like to extract those product keys but none of the "tools" or command line scripts I see online are working.

Try Showkey Plus from this forum:
ShowKeyPlus - Windows 10 Forums

If the old Windows 7 Product Key is in the registry from an upgrade, Showkey will display it. If showkey does not display it, then it is not currently available on the system. Also look for the Windows 7 COA stickers on the computer(s) with the product keys printed on them.

I tried Showkeyplus on my Win7 Pro machine. It came up with a Win10 Pro product key that is completely different than the Win7 Pro product key which the MS chat person somehow came up with.

Still wondering how did he get my original Win7 Pro product key? Or did he? Maybe he just pulled one from a list they keep or....

Ya that looks very similar to one of the screens he pulled up.

After he was done, I asked him to give me the Windows 7 Pro product key in case this happens again. He proceeded to paste a product key, along with the words "Windows 7 64 Pro product key", into a txt file on my desktop.

Product Keys mean little. MS use a hardware ID and that is why you had to activate again after changing hardware.
Something like a Network Card 'scores' highly, Graphics Cards 'score' less, but if done in close succession may result in a high 'score' and the need to re-activate Windows.

They were likely checking what you said about hardware changes was true, something like that.
It does not really matter what your Product Key actually is. It's a hardware ID and detected changes that matter.