Migrating mailboxes between Exchange 2007 and Exchange 2013

Mailbox migrations between Ex2007 and Ex2013 (in either direction) have to be done via PowerShell. Use the New-MoveRequest cmdlet, and provide the Mailbox and Destination Database:

New-MoveRequest -Identity 'Mailbox1' -TargetDatabase 'MailboxDatabase1'
To check the progress of your migration:

Get-MoveRequest | Get-MoveRequestStatistics