Ack! No, no, no. Call "them", and let them know that you need a new PNR built. If "they" won't do it, call Qantas or go to a travel agent. Make sure, though, that they cancel out your part of the itinerary on the first PNR, or it will auto-cancel when you miss your first flight. Could also cause problems with double bookings if any of your flights overlap. Finally, they might have built two PNRs to begin with, you need to make sure what sort of reservation you have.

Hagbard, a reroute is called a reroute because you are changing the routing of your ticket. If you don't change the routing, you don't need a reroute. These things are too much of a pain in the arse to go about rerouting willy nilly. When a reroute is done, other than the date of travel when things change a little, you can't really just reroute one part of the ticket. Or "rewrite" as you called it. The closest thing that they can do is a sticker endorsement. Otherwise, if one segment needs rewritten, grab a couple of mojitos because you will be there a while.