Convert Windows Server 2016 Desktop Experience to Core

In Server 2016 there is no longer a supported way to change to Core Edition, you have to make the decision which edition to use at install time.

I haven't seen any hacks yet to make this work, of course you can change the shell to cmd.exe or powershell.exe but I wouldn't recommend a manual removal of files and changing the registry.

So in your case I'm afraid you have to wait until ISOs are available on MSDN.

For those who always want to have statements backed up by vendor's documentation - statement can be found here and I quote it:

"Unlike some previous releases of Windows Server, you cannot convert between Server Core and Server with Desktop Experience after installation. If you install Server Core and later decide to use Server with Desktop Experience, you should do a fresh installation."