Originally Posted by JJeffrey

I suspect the fact that you were trying to just upfare the SJU-MIA segment confused things, just a guess. Typically airfares don't work like that. It's not possible to just upfare to business on one segment without splitting up the itinerary into 3 separate tickets and most likely costing way more, etc.

Nevertheless, if you search aa.com or Google Flights, can you buy a business class ticket on your flight date for the exact routing you're on? If yes, then definitely HUCA, the difference between what you paid and what the business class fare is currently is what you'll need to pay.