I'm using VMWare fusion to install Windows on my Mac for web testing. I already have Windows XP and a serial number for it. I have installed that. I also want Windows 7 to test in IE9+. If I buy Win7 Upgrade and upgrade it over one of my XP installs, will that void my XP serial? I'd like to be able to install XP again in the future using that serial number if necessary. Thanks!