Win7 to Win10, OS license usable for Windows 7 at later date? [duplicate]

If I upgrade from Win7 to Win10, can I transfer that key to Win7 at a later or is it locked into Windows 10 forever?

Background (Optional read): Outlook was crashing constantly the other day, ended up system restoring, fixed outlook but the PC is performing painfully slow now. I could upgrade it to Windows 10 (if it works it is software problem), or transfer the image to a new laptop (if it works it is a hardware problem). I'd like to know the answer to the question in case the PC is still slow after the 7->10 upgrade and I need to image the computer to the laptop.

Any answers or suggestions are much appreciated!

Solution 1:

When you upgrade a Windows version to the next, you are modifying/replacing your original licence agreement; it's not a new/separate licence. So you can't reuse the Win 7 licence you upgraded to 10 while you still have your Windows 10 installed.

If you remove the Windows 10 you upgraded to, you can reinstall 7 from scratch and use its key/license again.

Check out this related SU question: What happens to the Windows 7 key when upgrading to Windows 8?