Is it possible to switch between Windows 7 Editions?

Downgrading to a lower SKU is not possible, from what I know. Windows 7 Enterprise is a superset of Professional, so your only option is reinstalling, from my knowledge.

The same thing is true in Vista, you can only upgrade, not downgrade (Home Premium to Business was allowed, the other way around was not allowed).

Edit: here's the official response from Microsoft, courtesy of Mary-Jo Foley:

So the answer is no, you can't use WAU to move from Windows 7 Enterprise to Windows 7 Professional because that would essentially be a downgrade. Windows 7 Enterprise is just like Windows 7 Ultimate, all of the features are part of the SKU.

What WAU is meant to do is allow users to add more features to Windows 7 by upgrading to a higher edition without having to go through a time-consuming re-installation process. There are a variety of scenarios in which a user may choose to upgrade after the initial purchase, including:

1. A consumer purchases a PC pre-installed with Windows 7 Starter and wants to turn it into the best PC for entertainment.
2. A customer purchases Windows 7 Home Premium, but realizes they need Professional features including Domain Join, which provides the ability to add the computer to a company or school network.
3. Someone purchases a PC pre-installed with Windows 7 Home Premium and wants to experience of all of what Windows 7 has to offer with Windows 7 Ultimate.