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:
- A consumer purchases a PC pre-installed with Windows 7 Starter
and wants to turn it into the best PC
- 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
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.