Yes, anytime you make a change and your not on the exact same flight it will break the orginal PNR. The changing pasaanger will get a new PNR. You can still have the PNR's linked but this will not help with the seat selection/entitlement. The key is to keep the original passanger on the orginal PNR with no changes. I often ticket my daughter, her friend and I into coach and then call to purchase a Y-up for myself. Even though you on the same fligth you have to be in the same cabin to get non-staus companions the seats your seire. Any aircarft changes will still seperate your seating.