How to convert a full server 2012 to a core installation

To convert a full operating system installation to a server core installation using dism
Dism /online /disable-feature /featurename:ServerCore-FullServer
To convert a full operating system installation to a server core installation using powershell
Disable-WindowsOptionalFeature -online -Featurename Servercore-Fullserver
To convert a server core installation back to a full operating system installation using dism
Dism /online /enable-feature /featurename:Server-Gui-Mgmt /featurename:Server-Gui-Shell /featurename:ServerCore-FullServer
To convert a server core installation back to a full operating system installation using powershell
Enable-WindowsOptionalFeature -online -Featurename Servercore-FullServer,Server-Gui-shell,Server-Gui-MgmtFor servers with server core as the base installation perform the following steps to convert to a full operating system installation

Create an folder called C:\ws12\image
Insert the Windows 2012 media into the optical drive and type :
Copy install.wim from D:\sources\install.wim to C:\ws12\image
Run the following command to retrieve the name or index number of the image:
Dism /Get-ImageInfo /ImageFile:C:\ws12\image\install.wim
Run the following command to mount the offline Windows image (Note: index:2 = Standard Edition index:4 = Datacenter Edition) :
Dism /Mount-Image /ImageFile:C:\ws12\image\install.wim /index:2 /MountDir:C:\ws12\MountPoint
Upgrade the operating system by running the command:
Dism /online /enable-feature /featurename:Server-Gui-Mgmt /featurename:Server-Gui-Shell featurename:ServerCore-FullServer /source:c:\ws12\MountPoint\windows\winsxs
Run the following command to convert a full operating system installation back to a server core installation:
Dism /online /disable-feature /featurename:ServerCore-FullServer

For more information read this post.