In an Exchange cluster with a Database Availability Group (DAG), you can easily "switch over" one, more or all mailbox databases to another node.

Warning

If you need to take a node offline, it's better to put it in maintenance mode. This will properly notify the cluster that the node is in maintenance and prevents automatic failover (of other services, if any) to other nodes.

Switchover can be done through EAC, or through the EMS (Powershell).

To switch over all mailbox databases to a server called EX2:

Move-ActiveMailboxDatabase -Server EX2

To switch over the database called DB1 to a server called EX2:

Move-ActiveMailboxDatabase DB1 -ActivateOnServer EX2

The switchover will take a few minutes to complete, depending on the load on the database that needs to be moved. During the switchover, Outlook clients can lose their connectivity up to about a minute, but in normal circumstances, they should automatically reconnect again.

To verify the switchover (e.g. for a database called DB1), run the following cmdlet:

Get-MailboxDatabaseCopyStatus DB1

You should see the database Status of:

• On the active node: Mounted
• On all other nodes: Healthly

You can also see the active node for the database (e.g. DB1) with this cmdlet:

Get-MailboxDatabase DB1

The Server field will tell you the active node for the specified database. Leave the database parameter to see the active nodes (= Server) of all databases.

More information on mailbox database switchover can be found on Technet.