This sounds different of course but isn't "one pnr with multiple tickets" true for every trip that has changes applied? The change has a ticket number number and it's different from the original ticket number. It seems like sometimes the new ticket completely replaces the old one but at least sometimes it seems to represent only the change. I've gotten confirmation emails that only list the changed return flight and not the already flown outbound. And when I requested a receipt from AC I got an incredibly confusing series of documents, none of which had the complete final itinerary on them. As far as I can tell you have to have the whole collection of tickets that have ever been used on an itinerary including the original and all the changes and consider them all together to get the net price.