Travelling with a companion for a 1world rtw earlier this year, I could only secure one confirmed F class seat on our SCL-GIG flight, notwithstanding some full F fare seats still being available.

Luckily, since I was in charge of planning the trip, I secured the 1 available A class seat for myself. At SCL on departure, she was cleared from J to F, but it was kind of a hassle-- seemed like the ground staff weren't too used to last minute changes like this. We had to spend quite a few minutes at the check-in counter while at least 3 different people conferred, calls were made, etc.

At the end of the day, everything worked out fine. For me, it was a win-win situation as I was starting to tire of my travelling companion (indeed, she fled 2/3 of the way through the trip, complaining of exhaustion and exotic diseases of an undetermined nature).