Hi @Limiw ,
as an in-place upgrade from Windows Server 2016 to 2019 is supported I don't see a problem with SCSM and with a proper backup/restore strategy I would give it a try.
(https://learn.microsoft.com/en-us/windows-server/get-started/upgrade-2016-to-2019)
Just be aware Windows Server 2019 is not supported for SCSM 2019: https://learn.microsoft.com/en-us/system-center/scsm/system-requirements?view=sc-sm-2016#server-operating-system
If you are planing updates like this take care you have a proper backup/restore strategy for a rollback if something goes wrong!
Also setting up a test environment is a good option to test the upgrade procedure of OS and/or SCSM.
SCSM 2019 supports Windows Server 2016 and 2019. It might be a good option to update SCSM first from 2016 to 2019. If this is done you can update the server OS in the second step.
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Regards
Andreas Baumgarten