Delta has a very high change fee.
"Change Fees
Based on the fare rules, you may have to pay a service fee and/or a difference in fare. For travel within the 50 United States, the fee is $150 for Delta flights.
For travel outside the United States, the change fee is typically $250, but can vary based on location and type of fare. Changes are usually permitted only to the return portion of an international itinerary."
So even though your fare dropped by $100, as long as you have a nonrefundable ticket you'll still owe them $50.
You're only in luck if your ticket is refundable or you purchased it less than 24hrs ago.