You cant change IP address of ISP,
be sure to change ip address and subnet mask of LAN, they are related.
If you need to connect in multiple LAN at same time, go to http://download.cnet.com/IP-Changer/3000-2155_4-10065756.html
but whatever change u made on ur IP, it is local and doesnot affect ISP's gateway IP. That means u cant escape from FBI, if u are using torrent sites, hehe