From a console or the RLM/SP/BMC depending on the model so you don't lose access to the console when changing. You will need to edit rc and hosts with the new entries then you can manually enter the ifconfig to change the IP, then delete and re-add the default route after -- but all from a console unless you have another network interface setup since this one will go offline and won't have the right route when it comes back up.
If a cluster also look at the partner interface if needed.
What Scott said, plus the line you are looking for in the /etc/rc file is "ifconfig vif0 <stuff>"
Note that if this line reads "ifconfig vif0 `hostname`-vif0 <stuff>" it is assigning the IP address found in /etc/hosts to the VIF. Change the address in /etc/hosts on the <HOSTNAME>-vif0 line to the new address before you reboot. As you are changing the subnet mask you will need to edit the netmask section of the ifconfig line in /etc/rc.
You will still need to manually run ifconfig vif0 <NEW IP ADDRESS> netmask <NEW SUBNET MASK> to make the change to the live system