I have came across such a situation a million times where we move a machine from one IP to another (mostly when policy audits occur) and things stop working even though we modify DNS to reflect changes, just because some *beep* people still fiddle around with the old IP or some software were configured with IP instead of DNS (why? never got the answer, even though everyone knows DNS is here and hostnames are so much intuitive like a webserver having web.domain.com, an irc having irc.domain.com or in some cases chat.domain.com when we run multiple chat related tools like irc and jabber on the same server).
Everytime we moved a system to new IP, we would modify DNS and also modify firewall to redirect any connections to old IP to new. It wasn’t long but repetitive process mostly done through scripts. Today i found rinetd, something which i needed for years. Using rinetd you don’t need any firewall experience or etc. You just gotta edit one file and its all done.