Yes it is possible to perform with no strange tools at all. Before you start you need to have your product key (either a MAK or a KMS client key)
Start an elevated command or powershell prompt.
to get information on the current status.
dism /online /get-currentedition
to get the current edition, it should be an Evaledition
dism /online /set-edition:ServerStandard /productkey:XXXXX-XXXXX-XXXXX-XXXXX-XXXXX /accepteula
The key in this case is the KMS Client key for Windows Server 2016 and you can get all those keys from:
Restart the server (it will reboot twice)