Edit /etc/hosts, change the IP address corresponding to the system's hostname.
Edit /etc/netmasks, change the network and subnet mask.
If required, edit /etc/defaultrouter and set the new default gateway of the system.
Edit /etc/inet/ipnodes, change the IP address of the system's hostname. This file trumps all of the other TCP/IP config files, so if you miss this file everything else is pointless. Alternatively, you can delete or rename this file, and this will trick the Solaris 10 SMF subsystem into thinking it's an IPv4-only system.