SO we have options for making our process make them NON-PRIVATE if you wish.
OR if you want to migrate it as private and then change it, this VB script will help.
Sumatra Development leads the field of migrating entire calendar servers to Exchange. We migrate Oracle Calendar Server, Oracle Beehive, Apple iCalendar, and Zimbra to Microsoft Exchange keeping all meeting information and resource bookings intact. We migrate calendars server-to-server between Exchange and Office 365 while keeping meetings live and doing incremental syncs quicker than any other solution.