Your question can be answered many ways depending on if you want to change internal or external IP and how your network is set up. If you have the internet running straight from the modem to the computer then you have to put a new network card in so the modem sees a different MAC address. If you have a router you might be able to clone the MAC address of your computers network card and that may work. If you have a smoothwall then just change the RED and GREEN network cards with each other.
All is fair in love and war.