Fazer downgrade para um SKU inferior não é possível, pelo que sei. O Windows 7 Enterprise é um superconjunto do Professional, portanto, sua única opção é reinstalar, pelo que sei.
O mesmo acontece no Vista, você só pode atualizar, não fazer downgrade (o Home Premium to Business foi permitido, o contrário não foi permitido).
Editar : eis a resposta oficial da Microsoft, cortesia de 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 for entertainment.
- 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.
- 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.