If you ever have the situation where you are transitioning from one block of IP addresses to another because you are changing ISP's and you want to keep all of your services up and running while your domain name changes propagate check out:
rinetd - http://www.boutell.com/rinetd/
I tried for hours to see if I could get iptables to work but in the end the learning curve proved prohibitive for an single evening's investigation.
However, rinetd does exactly what's needed. You set up a box and bind your old IP addresses to it. You move your servers to the new IP addresses. Then you set up /etc/rinetd.conf to list out addresses and port mappings. It's the only port forwarder that I've been able to find that handles forwarding of separate IP addresses to different hosts from a single machine. If I hadn't found this I was going to break down and write it.
The config file looks like
a.a.a.a XX b.b.b.b YY
where a.a.a.a. is the old IP address bound to your forwarding box, XX is the port, b.b.b.b is the destination server and YY is the port you want it forwarded to. It took seconds to set up and seems to work like a champ in light testing.
We'll find out how well it works in production as this server (and many others) are going to have all their existing traffic moved through it.