(Note I will try to start posting again and address the Ultrafin issues. I have been doing a lot of studying as of late. I got my VCP5-DCV and am working on an MCSA 2012 upgrade. I am also working on an Exchange 2013 DAG. I hope I can post some tips once I iron out all the bugs).
Here is a simple but somewhat silly tip when you think about it.
In trying to decomission an Exchange 2010 server, I found I still had what seem to be system (arbitration) mailboxes in one of the mail stores.
I was in Exchange powershell on the 2010 server and was trying to move them to a 2013 mail store. I got an error with this message:
Unsupported target database version. The New-MoveRequest cmdlet can only move mailboxes to databases mounted on servers running one of the following versions of Exchange: Exchange 2010, Exchange 2007 (SP2 and later), Exchange 2003 (SP2 and later).
The fix is actually really simple. Run the same command from Exchange powershell on the 2013 servers.
Here is how to move all your mailboxes in one line:
get-mailbox -database 2010_db_name -arbitration | new-moverequest -TargetDatabase 2013_db_name
This will show you how the move is going: