The lowest fares require a 21-day advance purchase. Then there are 14-day, 7 day, 3 day, and walk-up fares. The answer to your question depends on how fare in advance you will know your schedule. If you will be making your final plans at least 14- or 21 days in advance, you should wait until you know for sure what you are going to do. If you are really making last minute (i.e., less than 7 days in advance) decisions, go ahead and buy them now, as the $200 change fee is less than the fare difference.