解决方法
根据微软,这是不可能的.来自TechNet：
Understanding Active Directory Domain Services (AD DS) Functional Levels：
When you deploy a new forest,you are prompted to set the forest functional level and then set the domain functional level. You cannot set the domain functional level to a value that is lower than the forest functional level.
Forest优先作为林中每个域的最低功能级别：
You can set the domain functional level to a value that is higher than the forest functional level. For example,if the forest functional level is Windows Server 2003,you can set the domain functional level to Windows Server 2003 or higher.
