How can the target users maintain access to their source mailboxes and other user's Outlook folde (4288610)

In a project where user accounts are migrated first, target users will need access to their source mailboxes in the same way it was setup on the source.  That way no business processes in organization are interrupted during the migration.  For example Administrative assistants will still need access to resource mailboxes to administer meeting requests or access to individuals Outlook Calendars to manage appointments.  How can this be provisioned for the target migrated users?