Here's some of what we told users:
Carleton College: Information Technology Services: GroupWise/Zimbra Compatibility
Carleton has long had multiple email directories, so the concept was not foreign to the users. I'm not the GW admin, but GroupWise here "always" had both the internal GW addressbook and and a "Carleton LDAP" addressbook to which all users had access. We simply had to tell them to use the latter rather than the former.
We were not able to figure out any way to script the setting of forward addresses in GroupWise. This seems to be a common question that, for more than a decade, has remained unanswered. Also, we have seen enough corruption of messages passing through GWIA that we didn't want to rely on it. So, we expired the GroupWise accounts (step number 7, at start of this thread) and instructed users remaining in GroupWise to contact
migrated-username@carleton.edu rather than
migrated-username@acs.carleton.edu. If GroupWise users sent mail to
migrated-username@acs.carleton.edu, then GroupWise would bounce it back to them with an "account expired" error.
This was only a problem for the two-week migration period, and only for GroupWise-to-GroupWise communication. The externally advertised MX for GroupWise was (and remains) a sendmail router under my control, and it was easy to script the changing of forwards there.