On your Domino Server you will have a Foreign Smtp Domain, Foreign Domain and a Global Domain. These are all used to set up the internet Connection. Normally there are no Ip addresses in these documents just Web Addresses.
If your External mail is working now all that will need changing when you change your ip addesses will be the IP Address on your Domino Server and the Default Gateway. This should then work.
The other issue are your users who will need there connection Documents to the server checking as this normally contains the ip address unless you are running wins and they contain the Server name.