Domain name change for branch office

[lanman]

I have an Exchange 5.5 system with two Exchange servers, one in city A and one in city B, using the Internet as it's backbone for messaging, hierarchy and directory replication. Currently, there are 2 internet domain names assigned: zhco.com at the home office in city A and sprfsp1.zhco.com in city B. This was setup temporarily as a means of getting messaging up and running when the two organization names were different. Recently I re-installed Exchange, using the same organization name as city A. The hierarchy and directory replication work fine, however I need to make everyone's smtp address work using only zhco.com and do away with the sprfsp1 part of the domain name in city B. Currently I have a connected site created in the IMS pointing all outbound mail from users on city A's server to sprfsp1.zhco.com. Mail sent to city B's users from outside the organization gets to city B's Exchange server by routing to sprfsp1.zcho.com setup at the ISP. How do get everyone under one domain name? Do I modify the existing connected site to city B and remove the "sprfsp1" part of the domain name and then allow all mail sent to city B's users to route through the city A Exchange server?