Install a KMS Key for Windows 8.1 or Windows Server 2012 R2 on previous KMS hosts

How to install KMS key for Windows 2012 R2 / Windows 8.1 on Windows Server 2008 R2 host (also for hosts with Windows 7, Windows 8, Windows Server 2012)

Install Updates to support W 8.0/8.1 WS2012/R2 activation

http://technet.microsoft.com/en-us/library/jj612867.aspx

http://support.microsoft.com/kb/2885698/en-us

Install .NET Framework 4.0 or 4.5 (VAMT Prep)

4.0: http://go.microsoft.com/fwlink/?LinkID=212547

4.5: http://www.microsoft.com/en-us/download/confirmation.aspx?id=30653

Uninstall previous versions of Windows Management Framework 3.0 if installed and install Windows Management Framework 3.0 (VAMT Prep)

http://go.microsoft.com/fwlink/?LinkID=240290

Windows6.1-KB2506143-x64 -> for WS 2008 R2

Install VAMT 3.1 (included in Windows ADKK) (VAMT Prep)

Download Windows ADKK for Windows 8.1 http://www.microsoft.com/en-US/download/details.aspx?id=39982

Now using "1. SLMGR" or "2. VAMT" to setup KMS ... I would prefer slmgr, because I had trouble using VAMT on Windows Server 2008 R2.

Use the WS2012R2 KMS host key on Windows Server 2008 R2, because W8.1 host key could not be added and you will get an error.

1. SLMGR

slmgr.vbs /upk (to uninstall old keys, do a restart if possible)
slmgr.vbs /ipk 12345-12345-12345-12345
slmgr.vbs /ato (do a restart if possible)

2. VAMT 

Start the installer and select VAMT and MS SQL Server 2012 Express checkboxes in the Wizard.

Configure VAMT and add a KMS Client Setup Key

Open VAMT and create a new database.

Install license keys and setup KMS host

Right click on the "Product keys" and select "Add product keys..."

Note: Please note that you can also use the slmgr.vbs /ipk <KMS HOST KEY> command as an alternative method to install the KMS HOST KEY.

Add the license keys into the prompted field,  then click on "Add Keys…"

Once the key has been verified, it will show up in the product keys overview.

Troubleshooting.

Error 0xC004F042 in Event 12290 appears on the KMS host and the clients couldn´t be activated. Maybe the KMS cached the old keys... and Windows 8.1 have trouble to activate

To resolve this issue I have done the following steps:
- Remove keys from VAMT.
- Type in the following command to uninstall the kms host key: slmgr /upk
- Restart the system
- Install the key using: slmgr /ipk 12345-12345-12345-12345
- Activate Windows using: slmgr /ato