changin IPs

We are changing our IPS and I need to know how to get the DNS changed?
We have a primary DNS server that is windows 2k I have set the dns to dynamic but have not seen any change in the DNS records. We have servile domain names and server and cant have a lot of down time if any. All of the servers have had their IPs changed through the advanced setting of the nic properties. How do you get the domain names to see both IPs or to get the Dynamic DNS working.