In Windows Server 2012 R2, you could choose to run a minimal GUI or a full desktop. Either way, the choice was yours to switch to Server Core at any time by removing the GUI features. Well, those days are no longer. Similar to Server 2008, with Server 2016 you must choose the type of installation up front. And if you don’t choose Desktop Experience, you will get Core:
There is no sanctioned way to revert back to Server Core if you’ve chosen Desktop Experience. If you compare the windows features list from 2012 R2 that have the word “GUI” in them, you’ll see that the 2012 choices are no longer available. So choose wisely!