For BA247 you may have more luck if you're willing/able to split the journey in GRU (the equipment stops there anyway). Service to EZE is only 4 days per week, and premium cabins either have lower allocations or sell out quicker on those through flights. GRU-EZE in any class you want is usually pretty easy on any day operates, since so many passengers disembark in GRU, so the trick is often to get an A seat (or a J/C/D - not sure which ones AONE6 access if A is not available) on one of the days that 247 goes onward from GRU to GIG, and then get a seat in A one or two days later onward to EZE.

Good luck!