Changing IP-s is sometimes a risky business. Especially if your server's behind a firewall, in another country, across a great body of water. So here's a little backup strategy to use in case something does go bad:
> sudo at now + 1 hour
>networksetup -setmanual "Built-in Ethernet" youroldip youroldnetmask youroldrouter
> Ctrl -D
That way, if all goes to hell, it should go back to the previous state after an hour. If all goes well, don't forget to
> at -l
> at -r jobnumber