It seems Microsoft is going to be breaking tradition when it comes to Windows Vista upgrades. With Windows Vista, users will not be able to use upgrade keys to initiate completely new installations.
Upgrade versions of Windows Vista Home Basic, Premium and Starter Edition will not install on any PC unless Windows XP or Windows 2000 is already on the machine in question. In recent years, upgrade versions of Windows could be installed on any PC.
So, this means you better plan on spending $80 more than you originally intended because the upgrade versions of Vista won’t except your old XP CD as proof that you really are just upgrading, you actually have to install Vista on top of XP.
According to Microsoft, you cannot use an upgrade key to perform a clean installation of Windows Vista because Windows Vista does not check for upgrade compliance. If you do not have a previous installation of Windows available, Microsoft recommends that you "purchase a license that lets you perform a clean installation of Windows Vista." Well of course they would recommend that.
Fortunately, the change will not mean that users cannot install Windows Vista to a new directory. Windows Vista’s upgrade process includes the option of backing up previous installations, and in some scenarios a "clean" upgrade is required.
Thanks to Ars Technica for the tip.