Product key for Windows
Product key for Windows
You can check your current product key by looking in your system settings or checking the installation files. Since you mentioned the box was thrown away, try accessing the registry or uninstalling the old version to see if any keys remain. If unsure, contact support for help recovering it.
You can turn off Windows on your original machine and then apply the product key to the new setup. Just ensure one system is active in deactivated mode.
It wouldn't function because moving a key between devices isn't possible.
I purchased my sister's Windows 10 key from an eBay seller with a good rating, around 5 euros per half year ago. He had sold many keys and seemed reliable. So far everything is fine, though I wasn't aware of any restrictions on the product.