It would make sense to think that when you start with a Windows Evaluation edition and you have spend a lot of time to make it work in your infrastructure and you are happy with it and what to use it, that it would be easy to change it into a regular version.
Something like: buy a licence, add the licence key and you are done.
But that is not how it works.
A Windows Evaluation server works for 180 days and after that it will simply stop. Trying to add a normal licence does not work. You have lost a server that if you have been uncareful, was used by a lot of people or systems in an organisation.
Luckily there is a way to make this right and change a Windows 2019 Evaluation version to a Windows 2019 Standard edition.
First, go to the Powershell and open as Administrator.
Then check to which types this version can changed into with:
dism /online /Get-TargetEditions
Changing types only works with a Generic key of the new type. You can find the generic keys of Windows 10 and Windows 2019 at https://www.tenforums.com/tutorials/95922-generic-product-keys-install-windows-10-editions.html
Then you change the windows type with
dism /online /Set-Edition:ServerStandard /ProductKey:N69G4-B89J2-4G8F4-WWYCC-J464C /AcceptEula
(Change ServerStandard with your preferred type and choose the corresponding ProductKey)
After that you need to restart the server. And you will have a changed Windows version, which is unlicensed. You can add a valid, licenced Product Key with
slmgr.vbs /ipk #####-#####-#####-#####-#####