Splitting a domain off one server to another

I need to "split up" an existing Exchange setup and need some advice.

Background:
1) Company ABC has their one Exchange server sending and receiving emails @abc.com.  Nice and simple.
2) Sister company XYZ gets created at another location. VPN gets set up between offices.  All computers, in both offices are on abc.com's domain, which is fine. I configure that same Exchange server to handle certain users to send/receive emails with @xyz.com.  Email traffic goes over the VPN.  They have a few servers (domain controller, file server, database server) and a bunch of workstations, all under the ABC domain.

Current situation:
Now, sister company XYZ needs to be _completely_ disassociated with company ABC.  They need their own Exchange server too.  Internally, the servers and workstations can still be on the ABC domain, but customers can't see that we have anything to do with each other.  Company XYZ has increased email traffic needs, so that's another reason to set up their own.  

The problem:
I can't seem to get the ABC domain out of the "Received" part of the email headers easily.  Example:

Headers in a sent message now:
Received: from emailserver1.abc.com ([167.x.x.x]) by mx.gmail.com (etc..)
Received: from workstation1 ([192.168.2.10]) by emailserver1.abc.com with Microsoft SMTPSVC;
Return-Path: <joe@xyz.com>
From: "Joe" <joe@xyz.com>
To: <testing@gmail.com>
Delivered-To: testing@gmail.com
Subject: test

How I would want them:
Received: from emailserver2.xyz.com ([65.x.x.x]) by mx.gmail.com (etc..)
Received: from workstation1 ([192.168.2.10]) by emailserver2.xyz.com with Microsoft SMTPSVC;
Return-Path: <joe@xyz.com>
From: "Joe" <joe@xyz.com>
To: <testing@gmail.com>
Delivered-To: testing@gmail.com
Subject: test

How would I get Exchange to report a different domain name without changing the domain on the server itself?  Even worse, the Exchange server is going to be on the same server as the domain controller, so I don't know if changing the domain is even an option.  And if I were to do so, would that screw up the user accounts and their workstation settings?  These things I would rather not find out the hard way.  I need to do as much as I can via remote control software too.

It would seem like the email headers are the only thing I would need to worry about here, unless I'm overlooking something.  Thoughts?

Thanks!