SJO is not a BF market, regardless of what aircraft operates it. It is a EUA-able flight.

If you mileage upgrade does not clear the seat will go out for EUA, and the chances of it flying empty are virtually nil. I'm not 100% sure on your priority, but I believe that some people say a mile upgrade is supposed to get priority at the airport for a battlefield upgrade, but I'm not sure that the GAs ever really get it right.