[Server 2022] DISM not showing ServerStandardCor as upgrade-able version

nsmeal 
I ran into the same problem and sadly, the usual suggestion is to try KMS keys (doesn't work) or to simply "reinstall everything". There's no comment from MS on this, officially the Upgrade from Server Standard Eval to Server Standard is supported but once you try this on the Core version, it doesn't work.

anyway ...
HERE'S THE SOLUTION:
1) get your hands on a non-eval ISO for server 2022 (some crafty google searches will get you there). Needs to be the same language as the installed eval version.
I'm not promoting this but if you happen to get a "dubious" download and want to make sure that ISO is legit and non modified, check its file hash (using PS> Get-FileHash "dubious-download.iso") against the database here: https://www.heidoc.net/php/myvsdump.php

2) On your target server (the one that declines the upgrade) open regedit, navigate to:
   Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion
and remove any occurence of "eval" (f.e.: EditionID = "ServerStandardEval" becomes "ServerStandard")

Should be about 3 entries or so.

3) Mount non-eval ISO and WITHOUT REBOOTING, open setup.exe, enter your product key, select standard core version (non gui) and proceed to do an inplace-upgrade. With above registry fix it will now give your the option to keep files and settings. let it do its magic and a few reboots later ... et voilà you're now running the non-eval version.

This worked even worked on a fully provisioned DC for me.

Disclaimer: I don't promote piracy of any form. Proceed with caution and always make backups (or checkpoints if you run this under hyper-v). Preferably test this on a non production clone.
Solution was inspired by: https://woshub.com/downgrade-windows-server-datacenter-standard-edition/