But be aware that even if you delete the OS off the drive, or even if you run the advanced commands to remove the Product Key from the install (slmgr -upk) nothing changes in the MS Activation servers until the Product Key is used to activate a new install. At that point if it's a retail Product Key the old hardware signature will be overwritten with the new one, a process that sometimes requires phone or agent activation and may be aided by the new Activation troubleshooter explained in
Reactivating Windows 10 after a hardware change.
Until a new hardware signature is assigned to the Product Key in MS Servers, the old hardware signature lingers simply because uninstalling the key via Command Line or deleting the old install does nothing to communicate to MS Servers.