Some questions about cross forest migration

Hi,

I have two Ad forests, each forest has its exchange organization: Org1 and Org2 (both exchange 2010)

I want to move my mailboxes from the org1 to org2. I read a lot articles about cross forest migration using ADMT and prepare move requests. And I have some questions about this process:

• Why is it necessary to migrate SIDHistory using ADMT?
• During the mailbox move, how can I manage coexistance  (email routing to non moved mailboxes)?

Thank you!