At a high-level, the impact of moving the mailboxes depends on what you do to prepare for it. Your Unity server which is provided Unified Messaging has a Partner Server with the mailbox server role installed. All messages route thru this partner server. If you move mailboxes to another server without giving Unity the proper permissions or integrating Unity with that mailstore, then you are likely to interrupt service and "break" Unity's integration with Exchange. When you are operating a Unified Messaging environment, there is a tight integration/dependency as well as a delicate balance between the Unified Communications world and the Microsoft AD/Exchange world.
I can be more specific if you give me some details on what you need and/or want to do.
The key text from the link I cited in my initial response:
Moving Subscriber Mailboxes for Cisco Unity 8.x
From time to time you may need to move subscriber mailboxes to another server that is faster or has more disk space available, or you may want to move mailboxes when you add new servers to your network. It is possible to move subscribers between servers without having to shut down Cisco Unity.
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