Before starting, let me inform you that Microsoft itself, promotes no way (or supporting) of downgrading Windows Server versions.
In my case, i wanted to downgrade several Windows Server Datacenter servers, to Standard Edition, during their in-place upgrade process.
The initial plan was to create new VMs and reinstall everything but the heavy configuration and the roles of each one of them, was putting way too much effort and man-hours to do so.
I spent several hours browsing the Windows registry and finally landed to the following path:
HKLM\Software\Microsoft\Windows NT\CurrentVersion At this point, i took a registry backup to be sure that anything gets harmed can be restored and proceeded to alter the below keys:
EditionID from ServerDatacenter to ServerStandard. ProductName from Windows Server 2008 R2 Datacenter to Windows Server 2008 R2 Standard.
Rebooted the VM and attempted to in-place upgrade to Windows 2012R2 Standard; Surprisingly, it worked and in-place upgraded the Operating System flawlessly. The same happened with in-place upgrading to Windows Server 2019 Standard.
Again, this is an UNOFFICIAL and UNSUPPORTED method by Microsoft, which in my case worked as expected.