Using iptables is there a way to switch the destination IP to become the new source IP and forward that connection.

Does iptables store the src and dst IP in a variable for a particular connection?

Have you tried MIRROR target?

For example:
iptables -I INPUT -s 10.0.0.0/8 -j MIRROR

You can read from iptables manual page:
iptables(8)
. . .
TARGET EXTENSIONS
MIRROR
This is an experimental demonstration target which inverts the source
and destination fields in the IP header and retransmits the packet. It
is only valid in the INPUT, FORWARD and PREROUTING chains, and user-
defined chains which are only called from those chains. Note that the
outgoing packets are NOT seen by any packet filtering chains, connec‐
tion tracking or NAT, to avoid loops and other problems.

"Works under Linux kernel 2.3 and 2.4. It was removed from 2.5 and 2.6 kernels due to it's inherent insecurity. Do not use this target!"

"Also note that outgoing packets resulting from the MIRROR target are not seen by any of the normal chains in the filter, nat or mangle tables"

I want to be able to also then do some POSTROUTING stuff on the packet.