When you setup the Exchange 2007 server you need to give the besadmin accounts rights to administer mailboxes on the server before you move any mailboxes using the command below.
add-exchangeadministrator <service_account_name> -role ViewOnlyAdmin
get-mailboxserver <server_name> | add-adpermission -user <service_account_name> -accessrights GenericRead, GenericWrite -extendedrights Send-As, Receive-As, ms-Exch-Store-Admin
Then you should stop the Blackberry services on the BES and replace the exchange system manager with the colloborative data object 1.2.1. This can be done before you setup the new exchange server as the files are backwards compatible.