That probably actually is the problem though. When Zimbra makes an appointment in your calendar it puts in an 'organizer'. When you change the username at all(prefix, domain, etc.) it changes the organizer and Zimbra assumes that you did not create the appointment as your name is now different.
Here's the bug link, so vote and it might get fixed:
Bug 7129 - cannot move existing appt/message/etc. to another user's mailbox
To check this go into Zimbra, download your ICS file ( http://<server>/service/home/<user>/calendar?fmt=ics ). Open the file in Notepad and do a search for your old and new email addresses. Likely what you'll find is that the old appointments have your old address associated with it.
Good luck, you'll need it.
I got some "support" on this issue awhile back, but it didn't help. As a result I just imported all my calendar entries to Outlook and then exported them to a new PST. I then ran the import wizard and put in my new email in the "change organizer..." box and then ran the import. The only problems I had were that I had to manually edit a bunch of the entries that weren't imported correctly.