How do I get H-Sphere working on two sets of IPs?

If you would like to ensure smooth change of IPs and have everything duplicated on the old and new sets of IPs before making the switch, you need to do the following:

On the Web box:

Before the IP migration you need to copy the /hsphere/local/config/httpd/sites directory to /hsphere/local/config/httpd/sites.old to preserve your old client’s apache configs.
Go to /hsphere/local/config/httpd/sites.old and edit index.conf changing sites to sites.old (cd /hsphere/local/config/httpd/sites.old; perl -pi -e ‘s/sites/sites.old/’ index.conf)
Copy namevh.conf to namevh.old.conf
Proceed with the IP migration.
Add the following lines at the bottom of the /hsphere/local/config/httpd/httpd.conf file:
Include /hsphere/local/config/httpd/sites.old/index.conf
Include /hsphere/local/config/httpd/namevh.old.conf
On the DNS servers:

Add your old DNS IPs to the /etc/named.conf config to force your DNS servers to listen to the old IPs.
Bind your old IPs to the NIC on your servers.