No first hand experience, so my advice may not be worth much, but even with a "flex" ticket you would almost with certain be subject to repricing of the ticket if you change the reservation, i.e. if the new flight is more expensive than what you paid, you have to pay fare difference. "Flex" usually means that you don't have to pay change fee (flat fee) but you always have to pay any fare difference regardless how flexible the fare conditions are.

What you could do is to arrive earlier at the airport and ask if you can standby for one of the earlier flight. Again, I have no first hand experience with this airline, but many airlines in South America will allow you to standby for earlier flight, i.e. you get on that flight if seats are available. No rebooking is made and hence no fees or upfare to be paid.