To answer to your original question here is a quick lab:
Generating traffic from R2 to R3 with DSCP EF (46), and doing a pcap on interface Port1 and Port2:
You can tell it´s the exact same packet as is has the same IPv4 identification 0x0023 (35), it´s just been nated as it exits port1, and it has kept the DSCP field.
The policy has diffserv-forward and reverse disabled:
diffserv-forward : disable
diffserv-reverse : disable
These commands are used if you need to remark the traffic, if you enable it, you´ll have the option to remark the traffic that hits the policy with the DSCP values you want.
Hope that´s helpful.