First, I initiated the removal of Exchange 2007 on the passive clustered mailbox role without any problem.
To remove the Active Clustered mailbox role, you must do the following:
Mailbox database properties--client settings--pointing the public database to the Exchange 2003 public database. This is not enough. Through the Exchange System Manager console, Exchange Administrative Group --Servers -- the Exchange 2007 Clustered Mailbox-role Server (here is CMS1) --public folders
Add the Exchange 2003 server as a replica to all public folders and remove the Exchange 2007 as their replica.
Still not enough, the uninstall process reports that there is public folders in the database.
By Exchange System Manager, add the Exchange 2003 server as the replica of Internet Newsgroups folder and remove the Exchange 2007 Clustered Mailbox Server from its replica.
Then, retry the REMOVE process.
