Just wondering why you'd require a MC install to upgrade to MC12. I have a new computer, and practically, space isn't really a consideration (2 500 GB hard drives), but (a) as a longtime user of PCs (I got my first one, an 80286, in 1990 a few months before my summer job at Microsoft), I know that few companies put much effort into the uninstall programs, leaving droppings all over the hard drive and in the Registry, and (b) why put users through such a PITA experience, especially when you're charging money for functionality that many other programs (Media Player, iTunes, and more) offer for free? (That said, I have long recommended Media Center, nee Media Jukebox, because I've found it to be a superior program, usability quirks notwithstanding.)
So if you're a verified user of a past version, why can't you just install MC12, enter that verification, and get the upgrade price? Why do you want to make it more difficult for me to give you money?