Hello,
Thank you for posting in Microsoft Community forum.
Based on the description, I understand your question is related to Server 2022 downgrade to Server 2019 OEM key.
Because OEM key is a Windows license that provide by the manufacturers, you can double check your OEM license manufacturer and contact them for detailed information about downgrade.
Besides, I noticed you said you have the same WS 2022 digital license bought from the O365 portal, and they have key conversion functionality to Windows Server 2019. This sounds good, if you encounter any issue with the license bought from O365 portal, please don't hesitate to seek help from O365 side in this forum. You can post under "Microsoft 365 and Office / Subscription, account, billing / For home / Windows" topic.
Have a nice day.
Best Regards,
Molly