see what Mike V. found
Scenario 3: Replacing Apple Mac operating system with (upgrading to) Windows. (This is not a common scenario due to technical difficulty, but it is allowed under the terms of Microsoft Volume Licensing.) If the customer has purchased a Windows 7 Professional Upgrade license for their Macs (without Software Assurance), the terms of the upgrade license require that they first remove the qualifying operating system (for example, Mac operating system) before they can deploy the Windows Volume Licensing Upgrade license and software (Mac operating system is a qualifying operating system in the Microsoft Volume Licensing program).
VLA will work (is legal) if you have SA or if you are ditching OSx (who does that?)
From what I understand ...