> The address of my old server I do not know.
That's annoying ... If it is a ftp server try a tcpdump -i any -n -v 'port 21' on the router and look at the destination address your device is trying to reach in the tcp SYN packets, and use this value as old-address in the iptables command.
> My new ftp would be like this:"abc.bplaced.net"
This address can be used as is or you can replace it with the result of nslookup abc.bplaced.net. So the command becomes:
iptables -t nat -A PREROUTING -p tcp -d old-server -j DNAT --to-destination abc.bplaced.net