Vista didn't quite allow that, but it did have a dumb workaround: you could still install Vista, but it wouldn't activate until you proved that you owned an old

Vista didn't quite allow that, but it did have a dumb workaround: you could still install Vista, but it wouldn't activate until you proved that you owned an older product. Well, from the unactivated Vista, you can run Vista setup, and "upgrade" on top of itself. That was enough for the OS to flag itself as being an upgraded install, and you could activate from that point.