If you accidentally gave the wrong key to Windows Licencing and upgraded your server from 2016 Standard to 2016 Datacenter, by definition found on Microsoft, you will have to reinstall and migrate services to freshly installed server.
But I found a simpler way. Go to – registry, run “Regedit.exe”
Edit: HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion
Change To: ServerStandard
Change To: Windows Server 2016 Standard
Run setup.exe from original Windows 2016 Install CD and choose the correct Standard version.
Chose upgrade and keep all files. After 10-15 minutes your server is running 2016 Standard.