If like me you decided to give Windows Server 2019 Evaluation Edition a try and then decided to buy a product key and need to switch editions on a live system then keep reading.
The easiest and simplest way to complete the switch to a Standard Edition Licence is to use Powershell and DISM.
Log into the Server and open PowerShell
type the following
DISM /Online /Set-Edition:ServerStandard /ProductKey:xxxx-xxxxx-xxxxx-xxxxx-xxxxx /AcceptEula
It will then ask you to reboot by typing 'Y'
After a reboot, open Powershell again and run the following command
DISM /Online /Get-CurrentEdition
All done :)