Using slmgr /upk only uninstalls the keys from Windows. It doesn't do anything with regards to being able to use the key on another machine. Iit simply removes the key from Windows, no effect on the activation servers
Do you have another key you plan to use on that copy of Windows?
Installing that key Slmgr /IPK sxxxxx-xxxxx-xxxxx-xxxxx-xxxxx should be just as effective.
If you are not planning on using that machine if you reformat the machine machine to remove Windows would again work just as well with regards to using /UPK
Running Windows after using /UPK means the machine is in an unlicensed state and doesn't know what features are available, so items may or may not work