Converting to Server 2012 Core from GUI is very easy. Microsoft said that 90% of their Windows Updates are GUI patches. Which is why they came out with Server 2008 Core. The only problem was that if administrators got stuck and could not find out how to accomplish a task via Command Prompt or PowerShell they had to uninstall the operating system. With the release of Server 2012 you can navigate between the interfaces.
Here is one my other blog posts for converting to GUI from to Core: https://crtech.tips/install-server-2012-core-gui/
If you would like to convert from GUI mode to Core all you have to do is uninstall the GUI features.
Type: Uninstall-WindowsFeature Server-Gui-Mgmt-Infra -restart
Note: Uninstalling the GUI features will require a reboot of the server
That’s it! The system will reboot without the GUI. Log in and you will be greeted with only a Command Prompt.