netfilter: Redirecting incoming udp packets to other port

Hi,
I want to redirect incoming udp packets comming to router to only
go out from a praticular port. will it be possible by using iptables
command. will the command change skbuff contents and tuple. Do reply
packets will be the routed properly.

scenario

client ------------> router -------------------->server
<----------- <--------------------


if receive a orignal udp packet from client from client port x to
router port y, can i use the same port y to forward to server (y acting
as source port) and then receive the reply packet to port y as
destination and successfully forward to client port x.

will the cmd like will work
iptables -A POSTROUTING -t nat -p udp --dport y --sport y -j MASQUERADE

can any one give idea

thanks in advance
murugan