Yes. In fact for some upgrades Microsoft requires a clean install of Win7 (but it is still classified as an upgrade). You are able to select clean install for any environment when starting the upgrade process. It may validate that the system has an eligible OS already installed (ie. you may not be able to do the clean install on a new bare drive -- that isn't quite an upgrade). Not sure if this is enforced diligently but I'm sure there are some eligibility checks.