Failed to submit the request because public folder migration has already been successfully completed previously

When migrating to Exchange 2013 modern public folders, you may want to repeat the migration especially if you need to tweak the Public Folder-to-Mailbox Mapping File and perform the migration again.

To repeat the migration progress you need to perform the following steps:

1. Removing all public folders from the newly created public folder mailbox with Get-PublicFolder –Recurse | Remove-PublicFolder (make sure you run this on Exchange 2013 Management Shell so you don't delete the public folders on your 2007/2010 environment).
2. Removing the default public folder hierarchy mailbox with Get-Mailbox –PublicFolder | Remove-Mailbox -PublicFolder
3. Clearing the MsExchDefaultPublicFolderMailbox attribute on the Exchange organisation with ADSIEdit as per Exchange MVP Satheshwaran Manoharan's article http://careexchange.in/how-to-recreate-public-folder-master-hierarchy-in-exchange-2013/
4. Remove the Exchange 2013 public folder move request using Get-PublicFolderMigrationRequest | Remove-PublicFolderMoveRequest

At this point you would think you should be ready to migrate the public folders to Exchange 2013 again however there is a final step which needs to be performed.  This final step has caught out many Exchange Admins and is not documented at all online.  Without the final step, the following error is experienced:

"Failed to submit the request because public folder migration has already been successfully completed previously"

This error occurs if the PublicFolderMigrationComplete attribute is set to $true on the Exchange Organisation Configuration as shown in the following screenshot.

Simply use the following PowerShell command to set the PublicFolderMigrationComplete attribute to $false.

Set-OrganizationConfig -PublicFolderMigrationComplete $false

After this final step you can go ahead and remigrate your public folders to Exchange 2013 from scratch.