This article shows how you can do port-forwarding with rinetd on Debian Etch. rinetd allows you to forward ports from one system to another. This useful if you have moved your web sites to a new server with a different IP address.
And another excellent guide at HowtoForge
Of course, you have modified your DNS records, but it can take a few days until DNS changes become effective, and that is where rinetd comes into play. If clients still use the old DNS records, rinetd can redirect them to the new server. With rinetd, you do not have to fiddle with iptables rules.