In Windows Server 2008 R2 is the ability Demote Forest Functional Level and Domain Functional Level.

Revert Back or Downgrade Forest Functional Level

Do this in certain circumstances that are explained below, we can do:

1 - Current Forest Functional Level should be Windows Server 2008 R2.

2 - Just can demote Forest Functional Level to Windows Server 2008.

3 - Optional features that can not be disabled, must not be activated.

For Example: Active Directory Recycle Bin.

Disable ADOptionalFeature

4 - We use Powershell command Set-ADForestMode.

(This is in Active Directory module for Windows PowerShell, Figure-1)

Active Directory Module for Windows PowerShell

Figure-1

5 - Format command is as follows:

Set-ADForestMode <Root-Domain FQDN> -ForestMode Windows 2008Forest

In this example Woodgrovebank.com is Root Domain FQDN.

Figure-2 below before the opration, as you see Forest Functional Level is on Windows Server 2008 R2.

Figure-2

Now we run the following command and then pressing Enter, type in the word Y and the Enter key again (Figure-3)

Set-ADForestMode Woodgrovebank.com -ForestMode Windows2008Forest

Figure-3

That was done and now we check Forest Functional Level (Figure-4).

Figure-4

Open Diagnostics ---> Event Viewer, and check Directory Services.

You have an Event ID number: 2040, with description New Forest Functional Level is 3 (Figure-5).

Figure-5

Number 3 is value for msDS-Behavior-Version Attribute (Figure-6).

Figure-6

Table value associated msDS-Behavior-Version.

• 0 = Forest functional level: Windows 2000
• 1 = Forest functional level: Windows Server 2003 interim
• 2 = Forest functional level: Windows Server 2003
• 3 = Forest functional level: Windows Server 2008
• 4 = Forest functional level: Windows Server 2008 R2 5 = Forest functional level: Windows Server 2012

Revert Back or Downgrade Domain Functional Level

Do this in certain circumstances that are explained below, we can do:

• 1 - Current Domain Functional Level should be Windows Server 2008 R2.
• 2 - Just can demote Domain Functional Level to Windows Server 2008.
• 3 - Should be Forest Functional Level on a Windows 2000 or 2003 or 2008.
• 4 - We use Powershell command Set-ADDomainMode. (This is in Active Directory module for Windows PowerShell)
• 5 - Format command is as follows:

Set-ADDomainMode <Domain FQDN> -DomainMode Windows2008Doman

In this example Woodgrovebank.com is Domain FQDN.

Figure-7 below before the operation, as you see Domain Functional Level is

on Windows Server 2008 R2.

Figure-7

Now we run the following command and then pressing Enter, type in the word Y

and the Enter key again (Figure-8)

SetADDomain Mode Woodgrovebank.com -DomainMode Windows2008Domain

Figure-8

That was done and now we check Domain Functional Level (Figur-9).

Figure-9

Open Diagnostics ---> Event Viewer, and check Directory Services.

You have an Event ID number: 2039, with description New Domain Functional Level

is 3 (Figure-10)

 Figure-10

Never forget, Revert back first Forest Functional Level,  then Domain Functional Level.

More info

AD-Powershell for Active Directory Administrators

Move (Transfering or Seizing) FSMO roles with AD-Powershell command to another Domain Controller