What do you get when you run;
Dism /Online /Get-TargetEditions
This one should work unless there's an issue with the key.
Dism /online /Set-Edition: <edition name> /AcceptEula /ProductKey:12345-67890-12345-67890-12345
Where <edition name> is the higher edition that you want to change to. (ServerStandard or ServerDatacenter)
The below thread discusses the same issue and you can try out some troubleshooting steps from this and see if that helps you to sort the Issue. https://social.technet.microsoft.com/Forums/windows/en-US/5418a7cd-23d1-48c3-acbd-9300211e2508/upgrading-from-windows-server-2016-standard-quotevaluationquot-to-a-fully-registered-version?forum=ws2016
I hope this information helps. If you have any questions please let me know and I will be glad to help you out.
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