like sbm12, I flew on CM last June. Upgrades on CM are notably easier, but I rode in Y because I wasn't elite. However, I am almost sure that at least on my PTY-SCL route that there were open seats in F. I was able to ride in 14A/F on all 4 segments (JFK-PTY-SCL and back), and that is probably the most desirable Y seat on the plane. In both directions, I did it just by asking at checkin 3 hours before departure.
Also, when considering upgrade chances, note that frequent fliers and status are generally uncommon in Latin America, so there is little competition.