As you know, in previous versions of Windows Server, changes to functional level on your organization could not be rolled back.If you raise your functional level from Server 2003 to Server 2008 or higher, you can not lower this level back.

In Windows Server 2008 R2 , this behavior has changed a little bit.Again you can not roll back functional level except one condition;

If you raise your functional level from Server 2008 to Server 2008 R2 then you can roll back to Server 2008 no earlier.