Eugene's observations

From Windows Vista to Windows 7

Microsoft will only allow the following upgrade paths to Vista users. For example, users who purchase an upgrade copy of Windows 7 Professional and have Vista Home Premium will only be able to perform a clean install. Here's the migration list:
Windows Vista Home Premium to Windows 7 Home Premium
Windows Vista Business to Windows 7 Professional
Windows Vista Ultimate to Windows 7 Ultimate

If you are performing an upgrade installation from Windows Vista, the base language must match the target Windows 7 language, you must have about 9GB of free space for the installation, and the installation can only occur on the same partition that holds Windows Vista. Existing applications and user data will be automatically migrated to the new Windows 7 installation.

Clean installs will require about 16GB for the installation process. Unless the user chooses to repartition or format the current partition, Vista's files from C:\WINDOWS will be preserved under C:\WINDOWS.OLD, just as when upgrading from XP to Vista. Clean installs will also have to be performed when upgrading from a 32-bit version to a 64-bit version.

Windows Anytime Upgrade (WAU)

Once you have some edition of Windows 7 on your system, whether you purchased it via an OEM or just upgraded from Windows XP or Windows Vista, you will be able to upgrade to a "more premium" version of Windows 7 by purchasing an upgrade key to unlock additional features, just as in with Vista. For example, you could start off with Windows Vista Home Premium, upgrade to Windows 7 Home Premium, and then later choose to upgrade to Windows 7 Professional.

As with Vista, the same DVD includes the various editions. Unlike with Vista, every edition of Windows 7 is a superset of the previous edition, so you will not lose any features when upgrading.