It looks like you are not retaining the same email address when migrating to the new system. Since the organizer of the appointment doesn't match the mailbox owner, you cannot edit the appointment (ie, only the organizer can edit it). If you did retain the email address or you modified the calendar data to set the organizer to the new email address, things would work as expected. There's a brief discussion of this here:
http://wiki.zimbra.com/index.php?tit...fter_Migration