Can I use my current copy of Windows 10 for a new build if I don't have a product key?

[Gcrona]

My computer was originally installed with Windows 7 and I later upgraded it to Windows 10 for free. I recently have been considering building a new PC and I wanted to know if there was any way I could use the copy of Windows 10 I have on my current computer and install it on to another computer. Or would I have to purchase a new key in order to install on a new computer?

 

[Colif]

Follow the steps here now and after getting new PC and you can move licence: Reactivating win 10 after new hardware

once moved it won't work on old PC but I assume you knew that.