How to Convert a Secondary Hierarchy Public Folder Mailbox to Primary Hierarchy in Exchange Admin Center (Exchange 2019)?

Hi Doumbia

Thank you for reaching out to Microsoft Q&A forum

Based on your description, I understand that you are managing an on-premises Exchange Server 2019 environment with public folders distributed across a primary hierarchy mailbox and multiple secondary hierarchy mailboxes. You want to promote one of the healthy secondary mailboxes to primary to avoid recreating the entire structure, while minimizing downtime, but your attempts with PowerShell commands like Set-OrganizationConfig -RootPublicFolderMailbox $null followed by reassignment and Update-PublicFolderMailbox -InvokeSynchronizer have not updated the status in the Exchange Admin Center (EAC), resulting in persistent secondary designation and intermittent user access issues.

From my perspective view, in order to minimize disruption in your Exchange 2019 setup.

1. First back up public folders with New-MailboxExportRequest, then check the target secondary mailbox's health via Get-PublicFolderMailboxDiagnostics <TargetMailbox> -IncludeHierarchyInfo.
2. Next step, you have to move content from the primary to the target using New-PublicFolderMoveRequest -Identity "\FolderPath" -TargetMailbox <TargetMailbox> .
3. Then remember to clear the primary role with Set-OrganizationConfig -RootPublicFolderMailbox $null (or delete msExchDefaultPublicFolderMailbox in ADSI Edit), wait 5-10 minutes for AD replication, then reassign: Set-OrganizationConfig -RootPublicFolderMailbox <TargetMailboxGUID> (get GUID from Get-Mailbox <TargetMailbox> | Format-List ExchangeGuid).
4. At the final step, you can force sync with Update-PublicFolderMailbox -Identity <TargetMailbox> -InvokeSynchronizer -SuppressStatus. Schedule during off-hours for any brief downtime.

Verification:

Run:

Get-Mailbox -PublicFolder | Format-Table Name, IsRootPublicFolderMailbox, $true

Then check EAC (Public Folders > Public Folder Mailboxes for "Primary Hierarchy"), and test Outlook access.

This summary is based on my findings from the community and several relevant threads. However, it may not accurately reflect the behavior in question. To help you reach your goal more effectively, I recommend posting a thread on the Microsoft Tech Community forum. It’s a great platform for deeper technical discussions and connecting with individuals who have hands-on experience and expertise. They’re best positioned to provide guidance and valuable insights on this topic.   

I'm hoping you'll get the help you need to fix the issue quickly. Please feel free to leave a comment below if you require any additional help. 

Best regards

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