Ler e seguir os passos indicados nesta página:
http://woshub.com/how-to-upgrade-windows-server-2016-evaluation-to-full-version/
Na prática para o Windows Server 2019 Standard ou Datacenter, fazer o seguinte:
Windows Server 2019: Upgrade Evaluation to Full Version
To convert Windows Server 2019 EVAL to a full edition, you need to use the GVLK (KMS) keys for Windows Server 2019. You can upgrade Windows Server 2019 edition the same way.
Convert Windows Server 2019 Evaluation to Windows Server 2019 Standard:
dism /online /set-edition:ServerStandard /productkey:xxxxx-xxxxx-xxxxx-xxxxx-xxxxx /accepteula
In order to convert Windows Server 2019 Evaluation to Windows Server 2019 Datacenter edition:
dism /online /set-edition:ServerDatacenter /productkey:xxxxx-xxxxx-xxxxx-xxxxx-xxxxx /accepteula
Confirm the command, restart the server. After rebooting, make sure your Windows Server Eval edition is converted to full retail.