Why do you want to change the IP adress it assigned. Are you setting up another computer that your changing to be the ICS server? If so then disable ICS in the one PC and add to the other. Not sure why you would want to change the IP address.
According to a website the following is stated about XP ICS
However, XP ICS is missing some features of those earlier versions. You can't disable the DHCP server, change the server computer's IP address, or change the range of addresses allocated by the DHCP server.
With that said it sounds like if you have an earlier version of Windows use that as your ICS if you want to change the IP. I hope this helped.