With the release of Windows Server 2012 it is now possible to switch between the GUI and Server Core installations using the GUI and Powershell cmdlets, with only a restart required to apply the changes.
The default installation of Windows Server 2012 is now available as Server Core, if you require to switch to the GUI installation of the operating system you can do using the following powershell cmdlets;
Install-WindowsFeature Server-Gui-Shell, Server-Gui-Mgmt-Infra -Restart
If you have a Windows Server 2012 GUI installation, you can switch to a Server Core installation, either by using the GUI to remove the Graphical Management Tools Infrastructure and Server Graphical components of the User Interfaces and Infrastructure feature or the powershell cmdlets.
Uninstall-WindowsFeature Server-Gui-Shell, Server-Gui-Mgmt-Infras -Restart