I'm having a similar issue - I booked a series of flights using Qantas points and one of the flights was LIM-EZE with LATAM. That flight has been cancelled and replaced with LIM-AZP. I think I paid for extra legroom seats (I know I did on our LAX-LIM leg), however the seat allocation has changed on the LIM-AZP flight. I have the pnr and putting that into the LATAM website it loads up the flights but I can't get the seat selection page to load for that flight now to have a look and also change the seats. Any advice?