Apparently, I didnt give zimbra (new server) and the old server time to do their thing.
The commands that i entered worked flawlessly, but when i tested it immediately afterwards (sending to the old domain from various addresses internal and external as well as from the old domain and the new domain) it wasnt working (the new server wasnt getting the emails directed to the old server)
I threw my hands up, went to lunch, and while i was out i got a call asking why email wasnt coming through (on the old server) and realized it was autoMAGICly being sent to the Zimbra server. YAY... Success.
Bottom line:
The line of code that i entered worked.
you have to find the domain ID# in the admin panel to use in the code
you have to wait a few minutes for the planets to realign properly
and finally...
it works.
Also... once the new zimbra server is up and running with all corresponding users, you DO have to set the MX records that point to your old mail server to the location of new mail server. If you dont, only LOCAL email will be forwarded to the new server. Once the MX records are updated, externally sent email will also get forwarded from new to old.

Thank you so much for your help!