1. Stop the Collaboration Services - Watchdog, Logging and Synchronization Services in the location where the mailboxes need to be moved.
2. Do a Local Move request of all the mailboxes to the new Store\Database
3. Start the Three services you stopped in Step 1.
4. Go into the properties of any subscriptions that have a Calendar Synchronization go to the Advanced Tab and change the server and Database to where the stub Mailbox Objects now reside.