Some might be surprised at the TechNet statement about moving mailboxes in Exchange 2013 that says:
In Exchange 2013 you must move mailboxes between Exchange 2007 and Exchange 2010 and Exchange 2013 from the Exchange 2013 admin center (EAC) and the Exchange Management Shell.
In fact, this is perfectly understandable and should not come as a huge surprise to experienced Exchange administrators because traditionally Exchange has always required that mailbox moves should be initiated from the higher version. Thus, to move mailboxes from Exchange 2007 to Exchange 2010, you initiate the move from Exchange 2010.
Why might this be the case? After all, Exchange 2010 CAS servers run the Mailbox Migration Service (MRS) too (MRS runs on the mailbox server role in Exchange 2013). MRS talks to MRS to move mailboxes from source to target database so it would appear feasible for an Exchange 2010 MRS to control the move.
That is, until you consider the very salient fact that Microsoft changes mailbox contents and settings from version to version. They do this to support new features or to accommodate issues that occur during migration. For example, the Exchange 2010 MRS contains some special logic to maintain UM settings during mailbox moves – something that an Exchange 2007 server knows nothing about.
The general rule is that a higher version of Exchange will always know about lower versions but it’s impossible for a lower version to have any knowledge about what’s coming in the future (none of us have this ability either). Once you understand this, it’s clear why a newer MRS will always control mailbox moves to ensure that mailbox content and settings are preserved accurately.
So when you’re planning your migration to Exchange 2013, make sure to remember that you always initiate mailbox moves (either individually or as part of a migration batch) from an Exchange 2013 server, using EAC or by running the New-MoveRequest cmdlet from EMS. It’s a simple rule to follow.
Follow Tony @12Knocksinna