From: Taylorville, IL
Well the simple answer is "yes".
The complex answer isn't so easy. We are not "there" and don't know all the details of what you have setup so exact details are impossible to answer.
A few things I can think of:
1. First (important that this is first!,...and Before! the move happens). Move all Mailboxes to the Exchange that you are keeping. Do it now while everything is intact and still working. Make the Exchange you are keeping the Primary for everything including being the Bridgehead. There is a LOT to doing this,...has to be done right the first time,..not the seconrd or third time. The company's Public MX Record needs to be shifted to reach the Exchange Server you are going to keep. Both Exchanges will still be running when you do this,...you are simply shifting roles and re-homing everything.
When the time for the move comes properly decommission that Exchange and remove it. The steps above are the prepraration for doing that.
2. Physically transport the DC over to the location of the new "Main Office" when the time comes. Adjust the AD Sites and Services Config in "Active Directory Sites and Services". DNS should automatically adjust for the new IP# of the DC, but double check it to be sure. Correct or delete any DHCP Scopes as required. Correct the static TCP/IP Specs refering to DNS on any equipment that requires this.
3. Don't waste your time trying to salvage the ISA. Rebuild it from scratch and forget it. You can export and import the Config to speed up the process but I probably would not even do that. The new location probably already has a firewall inplace,...just use it until you can get the ISA rebuilt.