Uninstall Windows 10, but save licence key

Windows 7 license key is printed in the box for Retail version. For pre-installed OEM versions, it is printed on a sticker on the PC.

If you have upgraded your license to Windows 10, you cannot get it in the registry using old tools because the activation process has changed.

Now during activation process, the license is sent with a numeric footprint of the PC's hardware to Microsoft servers and is not stored on the computer (only a generic key is stored : https://technet.microsoft.com/en-us/library/jj612867.aspx). The license is validated for a stored numeric footprint and Microsoft will always validate and activate this license to this PC.

By the way those tools can recover the license key from the Windows 7 installation. You may recover it if you have a backup (prior to the Windows 10 upgrade) that you can restore.