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