Late March (decision date) to late April (prospective travel date) means you can still meet advance purchase requirements for the cheapest fares. There may be limited availability. Given the price differential between lowest and flexible fares I would just wait to buy - unless you think there's a very significant probability (>50%) your travel will be interrupted. The change fee will be specified in the fare rules. ITA Matrix shows a $300 fee on a XK4X87M3 PAR to MIA fare.