As in Windows Server 2008 and Windows Server 2008 R2, Windows Setup in Windows Server 2012 allows you to choose one of two installation types : Server Core Installation or Server With a GUI (also called a full installation). One of the more interesting new features in Windows Server 2012 is the ability to convert a full installation to a Server Core Installation and vice versa, it was not possible in Windows Server 2008 / R2, and probably you know that Server Core was totally different of Full Version because Core version had a fewer features comparing with Full version.
You can switch between a Server Core installation and full installation in Windows Server 2012 because the different between these installation options is contained in two specific Windows features that can be added or removed. The firs feature, Graphical Management Tools and Infrastructure (Server-Gui-Mgmt-Infra), provides a minimal server interface and server management tools such as Server Manager and the Microsoft Management Console (MMC). The second feature, Server Graphical Shell (Server-Gui-Shell), is dependent on the first feature and provides the rest of the GUI experience, including Windows Explorer.
To convert a full installation to a Server Core installation, just remove these two features in Server Manager. Note that removing the first feature will automatically remove the second.
This first example I will remove only Graphical Management Tools and Infrastructure
After rebooting you can see this interface, called minimal server interface, SCONFIG command interface behind and Dashboad (ServerManager) in front.
Now when I remove the second option, Server Graphical Shell and after rebooting
I can see only prompt command, execute C:\PowerShell and open PowerShell command, execute C:\[PS] Get-WindowsFeature and take a look User Interface and Infrastructure session, all sub session will be unchecked.
Probably you are thinking that is possible to convert a Core to Full the same way, yes, you are right, in fact only install this two feature again and return a Full Version quickly ! Good Luck !