I was on a MIA-LIM flight last week in J. For some reason they had to change planes from the usual LP three row J configuration to the five row seating. This meant that many Y seats disappeared. They had to move passengers from Y to J. Luckily the seat next to me had been blocked at the counter, but all the other J seats but one were filled. The FA told me that the upgrades were given depending on which fare had been paid.