If your flight price decreases within 14 days of booking you will receive a credit for the difference back to your travel bank, and those funds can be applied to a future reservation. There is no $100 change fee if you are within that initial two week period. All you need to do is call.
I booked flights for October on March 8th of this year. 8 days later, one leg of those flights went down by $25 per person. On the original booking, one of us paid cash and one of us used Jetblue points. On the cash booking, the $25.03 was deposited into a travel bank for my DH. On the points booking (for my seat), the original transaction was reversed and reprocessed, with the difference in points being redeposited to my True Blue account. All it took was a 15 minute phone call.