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After you've migrated the box, you can contents/service from the old box to the new box, kill any service that it used to run on the old box, then run this simple iptables script on the old host, which will forward all traffic to the new host:

#!/bin/sh
 
NEW_IPv4="198.51.100.2"
NEW_IPv6="2001:db8::4"
echo 1 > /proc/sys/net/ipv4/ip_forward
echo 1 > /proc/sys/net/ipv6/conf/all/forwarding

for PORT in 25 80 443; do
 iptables -t nat -A PREROUTING -p tcp --dport $PORT -j DNAT --to-destination $NEW_IPv4
 ip6tables -t nat -A PREROUTING -p tcp --dport $PORT -j DNAT --to-destination $NEW_IPv6
done
iptables -t nat -A POSTROUTING -j MASQUERADE
ip6tables -t nat -A POSTROUTING -j MASQUERADE

Voila - transparent IPv4/IPv6 forwarding. The only "downside" is that you will see the IP address of the old box in the access logs of the new box.

Leave this running for a day or two, and keep an eye on the logs. Once you're certain that no traffic come in to the old box, kill it.

Note that the IPv6 part requires at least iptables 1.4.17, which means the OS should be at least RHEL7/CentOS7, Debian 8, or Ubuntu 14.04 LTS.

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