Those of you who have paid Microsoft a license to have Windows legally know what a product key means.
If you ever want to move Windows to a new computer and don't plan to useste the old one, you don't need to pay another operating system.
If your license is not OEM but Retail you can enable the old copy of Windows on your new computer.
The simplest way to turn them on Windows 10 is to sign in to your Microsoft account. In case you don't know, your Windows license is tied to your account with the company. To validate the product key, the operating system sends some information on Microsoft servers. The data these are unique to your computer, (processor and motherboard information). So if you change hardware on your computer, i.e. a new CPU or motherboard, the Windows you are running will probably be disabled.
So if you can't activate your operating system by signing in to your company account, you'll need to use your Windows 10 product key. Some (OEM) computers probably have the key on a sticker. If it is indiscernible there is the ShowKeyPlus.
ShowKeyPlus is a free tool that finds your Windows product key. Run the program and it will show you the operating system name, product ID, the version and the key. It also shows you if your license is OEM.
Click save and ShowKeyPlus will save the license data in a text file. You may find this information useful in the future.
Download the application from the Microsoft Store