Changing IP address Range on W2k Network

We have an W2K SBS server setup with a default 192.168.xx.xx address range. It currently uses an IDSN router through a 2nd NIC.
We now have ADSL installed and the ISP has assigned a range of IP addresses, which they said was needed for Exchange to communicate. Can I just modify the IP address of the 2nd NIC or do I alter all the IP addresses.
If altering is the answer I can do DHCP OK but what about DNS and what other places on SBS?
TIA