The first Exchange 2010 server in our 2007 org was a combination mailbox CaS & Hub server. We now would like to use this hardware as a dedicated UM server and I need to remove the mailbox role. (CAS & HUB are already gone.) All that's left is removing/moving the arbitration mailboxes, deleteing the databases and using Setup to remove the role. Can I delete the two system mailboxes and the Federated mailbox is see when I use this Powershell: "Get-Mailbox –Database “Mailbox Database 1073614215” -domaincontroller "RootDomainDC" -Arbitration | ft -wrap -auto" or do I need to move them? Is it important where we move them? (I have to use the -domaincontroller parameter to see the mailboxes, we have a placeholder root domain with several children and the mailboxes do not show if you don't use it.) Thanks!

Move them to another store. Any good candidate will do :) Set-ADServerSettings -ViewEntireForest $true will set your scope to the entire forest so you can see them