Before removing the last legacy Exchange server from the organization during an Exchange Server 2007 transition you should move the Public Folder hierarchy from the Exchange 2003 Administrative Group to the Exchange 2007 Administrative Group.

Right-click the Exchange Administrative Group (FYDIBOHF23SPDLT) and choose New -> Public Folders Container.

Now simply drag the Public Folders tree from the Exchange Server 2003 administrative group to the Exchange Server 2007 administrative group.