Okay, so some piece of code :)
First we'll need to allow IP forwarding.
echo 1 >/proc/sys/net/ipv4/ip_forward
Add a PREROUTING rule in Iptable to redirect everything coming from port 80 to the new server (replace by your own value of course)
iptables -t nat -A PREROUTING -p tcp --dport 80 -j DNAT --to-destination 255.255.255.128
Then we add an extra POSTROUTING rule to make the request to the new server coming from your client and not the old server
iptables -t nat -A POSTROUTING -p tcp -d 255.255.255.128 --dport 80 -j MASQUERADE
And that's it !
if for any reasons you have to remove these rule, follow these steps
List all the PREROUTING and POSTROUTING rules
iptables -L -vt nat --line-numbers
You get your previous rules with a line number. By using this line number you can delete the wanted rules (replace line-number-x by the right line number)
iptables -t nat -D POSTROUTING line-number-x
iptables -t nat -D PREROUTING line-number-x
Finally block the IP forwarding
echo 0 >/proc/sys/net/ipv4/ip_forward