Site database upgrade from Xendesktop 7.15 to CVAD 1912 fails with below exception, 

Error Id: XDDS:346DA064
Exception:
    DesktopStudio_ErrorId : DatabaseConnectionDetailsAlreadyConfigured
    Error Source : CitrixDelegatedAdmin
    Sdk Error Message : Cannot transition directly from using one data store to another; clear the existing data-store connection first.
    Sdk Error ID : Citrix.XDPowerShell.Status.DatabaseConnectionDetailsAlreadyConfigured,Citrix.DelegatedAdmin.Sdk.DataStore.Commands.SetAdminDBConnectionCommand
    ErrorCategory : NotSpecified
    DesktopStudio_PowerShellHistory : SimplePowerShellScript
    8/22/2020 11:09:42 PM
 

To resolve this issue, set the Configuration Logging to 'Disable' or Enable' but not ' Mandatory' for the time of the upgrade.

To set configuration logging to enabled, run the below powershell commands as an admin and make sure it’s replicated to all controllers in the site:

asnp citrix*

Get-LogSite

Set-LogSite –State “Enabled” or Set-LogSite –State “Disabled”

Problem Cause

Upgrade fails when executing the powershell script SetDBConnection with exception DeniedByConfigurationlogging. 

The error "the database has already been configured"/” Cannot transition directly from using one data store to another; clear the existing data-store connection first” indicates that the connection strings for some of the CVAD services have not been configured (set to null) during the upgrade process for some reason.

The issue happens due to the Configuration Logging feature set to mandatory.