Sending mail from secondary server to primary

[ryanenation]

We're currently rolling out a new Zimbra server to replace our existing email server running Postfix. We have configured a split domain with the Zimbra server as the primary server (new.example.com) and our existing server as a seconday system (old.example.com). We've created all necessary accounts and issued the following zmprov command for each to forward email to the secondary system:

zmprov ma user1@example.com zimbraMailTransport smtpld.example.com

All of that is working great. The problem arises when we start moving accounts from the secondary machine to the primary machine. We use the following command from the Split-Domain Wiki:

zmprov ma user1@example.com zimbraMailTransport lmtp:new.example.com:7025

After issuing the command all outgoing email from user1@example.com on the new server is delivered correctly, but the following scenarios occur when receiving email:

someone@yahoo.com -> user1@example.com => new server
user2@example.com -> user1@example.com => old server

In the above scenario, user2@example.com has not been moved to the new Zimbra server. Is there any way to enable accounts not yet moved to the new Zimbra server to deliver mail to accounts that have been moved? I haven't tested it yet, but I have a hunch that removing the moved accounts from the old server may do the trick, but I'm reluctant to do this in case we need to revert back to the old server for some reason. Is there another solution I'm not thinking of? We don't have the resources to move everyone at once so during the transition I need a workaround.

Ryan