If the upgrade space is not available at time of booking on an expensive DL fare to EZE, then you are gambling with DL and have risked wasting money. That may well mean buyer's remorse.

I have better luck with upgrades to/from EZE at around this time of the year with AA than with DL, but it's not unusual for them to clear within 0-48 hours of departure at this time of the year. It's also not unusual to find customers who wasted money buying a much more expensive upgradeable fare that ends up not being upgraded on the long-haul segment.