A: Using the Uninstall-WindowsFeature and Install-WindowsFeature cmdlets makes it easy to add and remove the components for the Windows Server 2012 (formerly code-named Windows Server 8) configuration level desired, as I explained in my FAQ "Q: Is it true Windows Server 2012 allows a server to switch betweenServer Core and Full Install mode without reinstalling the OS?"
The two features you will be adding or removing are Server-Gui-Mgmt-Infra for the management tools and Server-Gui-Shell for the graphical shell.
To take a full server and make a Server Core installation, use the following command. Note that it's not required to specify the Server-Gui-Shell to be removed, as removing the Server-Gui-Mgmt-Infra forces the removal of Server-Gui-Shell as it is dependent.
Uninstall-WindowsFeature Server-Gui-Mgmt-Infra –Restart
To take a Server Core and make it a full server with a GUI, use this command:
Install-WindowsFeature Server-Gui-Mgmt-Infra,Server-Gui-Shell –Restart
Likewise, you can just add Server-Gui-Mgmt-Infra and not Server-Gui-Shell to make a minimal server installation, or you can just add Server-Gui-Mgmt-Infra to a Server Core to make a minimal server installation:
Uninstall-WindowsFeature Server-Gui-Shell –Restart
Install-WindowsFeature Server-Gui-Mgmt-Infra –Restart
See more Windows answers (plus virtualization, System Center, and more) here: John Savill's FAQS.