The only way you can roll back your forest functional level is from Windows Server 2008 R2 to Windows Server 2008 if Active directory Recycle Bin is not enabled. Review the following guidelines that apply to raising the domain or forest functional levels:

• You must be a member of the Domain Admins group to raise the domain functional level.
• You must be a member of the Enterprise Admins group to raise the forest functional level.
• You can raise the domain functional level on the primary domain controller (PDC) emulator operations master only. The AD DS administrative tools that you use to raise the domain functional level (the Active Directory Domains and Trusts snap-in and the Active Directory Users and Computers snap-in) automatically target the PDC emulator when you raise the domain functional level.
• You can raise the forest functional level on the schema operations master only. Active Directory Domains and Trusts automatically targets the schema operations master when you raise the forest functional level.
• You can raise the functional level of a domain only if all domain controllers in the domain run the version or versions of Windows that the new functional level supports.
• You can raise the functional level of a forest only if all domain controllers in the forest run the version or versions of Windows Server operating system that the new functional level supports.
• You cannot set the domain functional level to a value that is lower than the forest functional level, but you can set it to a value that is equal to or higher than the forest functional level.
• With versions of Windows Server that are earlier than Windows Server 2008 R2, you cannot roll back or lower a functional level under any circumstances. If you have to revert to a lower functional level with a version of Windows Server that is earlier than Windows Server 2008 R2, you must rebuild the domain or forest or restore it from a backup copy.
• After you set the domain functional level to a certain value in Windows Server 2008 R2, you cannot roll back or lower the domain functional level, with one exception: when you raise the domain functional level to Windows Server 2008 R2 and if the forest functional level is Windows Server 2008 or lower, you have the option of rolling the domain functional level back to Windows Server 2008. You can lower the domain functional level only from Windows Server 2008 R2 to Windows Server 2008. If the domain functional level is set to Windows Server 2008 R2, it cannot be rolled back, for example, to Windows Server 2003.
• After you set the forest functional level to a certain value in Windows Server 2008 R2, you cannot roll back or lower the forest functional level, with one exception: when you raise the forest functional level to Windows Server 2008 R2 and if the Active Directory Recycle Bin is not enabled, you have the option of rolling the forest functional level back to Windows Server 2008. For more information about the Active Directory Recycle Bin, see What's New in AD DS: Active Directory Recycle Bin (http://go.microsoft.com/fwlink/?LinkId=141392). You can lower the forest functional level only from Windows Server 2008 R2 to Windows Server 2008. If the forest functional level is set to Windows Server 2008 R2, it cannot be rolled back to Windows Server 2003, for example.