The solution turned out to be a setting in the phones calendar retention setting on the phone itself. The phone was set to only retain the last 60 days worth of calendar entries. Setting the retention setting to "Forever" prevented it from both deleting the old calendar entries in the phone and from deleting the entries in Outlook during the sync operation via the Blackberry Desktop software.
The old Blackberry phone did not cause old entries to be deleted in Outlook during sync.