Exchange 2010 switch all mailboxes over to other server

This applies to DAG's where you have 2 mailbox servers

Open Exchange Management Console 

Under Server Configuration > Mailbox right click the server you want to switch the mailboxes from and click switchover server 

In the pop up that appears tick use the specified server and click browse. Double click the server you want to move the databases to. Click Ok. 

Give it a few minutes then close and reopen the EMC to check all mailboxes show mounted on the server you want them on. It should show healthy for the other server unless there are problems.