If/When this happens to you, have the agent look up the "Guaranteed Airfare" clause in the fare rules. It explicitly states that if the fare goes down on an itinerary identical to yours then you are entitled to the difference between the lower price and what you paid, with that difference being remitted to you via voucher (for nonrefundable fares) or your original form of payment (for refundable fares). There is no change fee involved, so have them look up "Guaranteed Airfare" or GAF the next time they give you this excuse.