Hi @andrey polyakov ,
If you want to downgrade an installed Windows Server 2019 to Windows Server 2016, you will have to reinstall the operating system.
For the license downgrading rights, please refer to the official documentation below:
Downgrade rights for Microsoft Commercial Licensing, OEM, and full-package product licenses
https://download.microsoft.com/download/3/D/4/3D42BDC2-6725-4B29-B75A-A5B04179958B/Licensing_brief_PLT_Downgrade_Rights.pdf
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, thank you!
Best regards,
Leon