I recently migrated a user on Outlook with a pop account (PST file) to Exchange (OST file). We need to import his contacts from the PST file to the new Calendar in his Exchange box.
I added the PST folder back into Outlook and did a file export of just the calendar items to a new PST file. I then used Import to import that file into the Calendar folder in his Exchange mailbox. The import routine complains that the import file doesn't contain calendar data (the file is 5 mb).
I used this method on another Outlook user going from POP to Exchange with no problems.