I have a somewhat unusual Exchange to Zimbra migration problem.

I have a group of users (around 50) who are on an Exchange server under our primary domain. Roughly 40 of those need to convert their calendars, email, and contacts to Zimbra. The problem that I have is that the remaining 10 are in a different department and will not be converted to Zimbra. All of the users have calendar meetings in Exchange that exist regardless of who needs their calendars migrated.

Some of the meetings in the calendars will need to stay able to be accessed by both Exchange and Zimbra users. Is there any tool I can use to keep from breaking the calendar appointments for everybody--or some way to set up aliasing across the calendar so everyone gets properly recognized and notified if there are schedule changes?