One disadvantage of Server Core with versions prior to Windows Server 2012 was an option was not available to switch back and forth; once a server was configured to use Server Core, it had to be re-installed to restore the GUI. Windows Server 2012 introduces the option to enable or disable the GUI with a reboot. A few different methods are available, but below are the PowerShell commands to accomplish the task.
Either line below will remove the GUI from a non-Core installation of Windows Server 2012. A reboot will be required.
Remove-WindowsFeature Server-Gui-Shell, Server-Gui-Mgmt-Infra
Uninstall-WindowsFeature Server-Gui-Shell, Server-Gui-Mgmt-Infra
Either line below will restore the GUI with a Core configuration. A reboot again will be required.
Add-WindowsFeature Server-Gui-Shell, Server-Gui-Mgmt-Infra
Install-WindowsFeature Server-Gui-Shell, Server-Gui-Mgmt-Infra