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, Canada, Puerto Rico and the U.S. Virgin Islands, the fee is $200 for Delta-marketed flights.
I’m sure American will follow shortly as well. I think it’ll be interesting to see what the non-major US legacy airlines do, like Alaska. They currently charge a $100 change fee ($75 if done online), so will they increase their change fees while still undercutting the legacies, or are these round of changes only for the big four (well, big three now, I guess)?