On this specific route, the fare classes which the lowest FUN (E class) and ECONO (L class) are booked in pretty much 'move together' - meaning if there are no E class, then there are no L class. It will be more expensive to upgrade an ECONO fare than a FUN fare, also ECONO carries a higher change fee - bsaed on that, your best choice is to book FUN in the future.