Switch to Windows Core with GUI:
Import-Module ServerManager Uninstall-WindowsFeature Server-GUI-Shell -Restart
Switch to Windows Core:
Import-Module ServerManager Uninstall-WindowsFeature Server-GUI-Mgmt-Infra -Restart
Switch from Core to full GUI:
Import-Module ServerManager Install-WindowsFeature Server-GUI-Mgmt-Infra Install-WindowsFeature Server-GUI-Shell Restart-Computer