Following a bit of a discussion on the user groups around this area, I thought I would add my thoughts here.

Many home PC users will be getting Windows Vista Home Premium and then potentially discovering that they really wanted the business edition (SKU) as they are not just home users.  The problem we have is that if you look at the Windows Vista product feature matrix you will notice item like those to the left (click to see larger image) where features are available in the Home edition, but not the business edition.  When people "upgrade" within the same release of the OS they expect to not loose features and some of those features include items that Microsoft potentially pays licensing for.  Because of these reasons, home users can either "upgrade" to Ultimate, or buy Business in retail and do a full clean install.

Now, if someone has any version of Windows XP or Windows 2000 then they can "upgrade" to any of the Windows Vista editions, however the key item to note here is that if someone will loose functionality, they will end up doing a clean install (by running from the existing install of Windows) of Vista.  This is detailed at Upgrade Paths for Windows Vista and a link to the key table is provided on the right.

Hopefully this makes it all clear, but if not, drop a comment on and I will respond.

ttfn

David