I haven't done EZE, but i have done GRU/GIG three times in the last 9 months and my experience is that the upgrades clear very late, if at all. In GRU, that has been a problem because UA outsources ground services to a 3rd party who has NO CLUE about how upgrade protocols on UA work. Very frustrating, I am usually pretty confident in "at the gate" upgrade situations everywhere except GRU. This last time - maybe 6 weeks ago - it seemed to be UA personnel and - lo and behold - i cleared at the gate.

However, with as many seats open as you describe, my guess is you have a pretty good chance. I somehow doubt that there is a huge population of displaced VG flyers out of EZE

On the 767s, getting Row 16 on the sides is pretty useful - a ton of legroom and you can put a bag under your feet. That - and an ambien - can make the time pass fairly quickly.