New Forest. I migrated the user accounts over and kept their SID history and am manually moving their mailboxes/adding them to the new BES while we migrate their workstations over.

EMS Command:
move-mailbox -TargetDatabase "exchange.newforest\storage group\mailbox database" -Identity 'mailbox name' -GlobalCatalog dc.newforest -SourceForestGlobalCatalog dc.oldforest -NTAccountOU "OU=XXXXX,DC=newforest" -SourceForestCredential $c -TargetForestCredential $t