Having given up on Easter Island, I noticed that Lan provide F service to GRU and GIG. After a long conversation with reservations, they agreed to book me in to C as there was no D left.
Upon re-issuing the ticket (took 90 minutes as this trick, I now know, is that the fare must be converted in to US$ before LA can re-issue. There´s a hint for everyone), the ticket staff sat me in F as I had a RTW ticket, eventhough it should have been A.
At check-in

LA750\751 SCL-GIG (via GRU)
The aircraft had been changed for a LanExpress 767 so no F!
The provided a downgrade voucher and offered a window seat with the one next to mine kept empty. They did not manage to keep this last promise!
Flights were fine, on time, and the staff at GIG provided another downgrade voucher. I feel lucky to have got these as there is nothing in rules requiring LA to provide them to me.

LA600 SCL-LAX
Is now delayed 4 hours for no reason that anyone can explain! So I guess LA had the last laugh on me!