Paul Thurrott posted a guide about upgrading from a Higher-End Vista/XP Version to a Lower-End Windows 7 Version:

Should be easy, right? Should be, but isn't. Here's how it works. Anyone using a valid, activated version of Windows Vista (or XP) qualifies for any Windows 7 Upgrade version. But in order to perform a "true" (or "in-place") upgrade, you must be upgrading to an equivalent, or higher-end, Windows 7 version

Looking at this chart, you can see that an in-place upgrade from, say, Windows Vista Home Premium to Windows 7 Home Premium is perfectly acceptable. As is an in-place upgrade from Windows Vista Home Premium to Windows 7 Professional or Ultimate.

But what if you want to upgrade from Windows Vista Business to Windows 7 Home Premium? In that and other similar cases, an in-place upgrade is impossible. So you will have to perform a migration,