I want to buy a Windows 8.1 Licence and upgrade to Windows 10. However from time to time, I need to format my PC and do a clean install (or I may just buy another PC). What I want to know is, If I do this, Will I be able to reinstall Windows 10 and use the 8.1 Licence? Or Will I then have to buy another Windows 10 Licence? Thanks in advance for any help.