Heck: That's just the "change" fee. You will still have to pay the difference in the current cost of the new ticket. This can end up costing you A LOT of money.
Last January we wanted to extend our time in Florida. Called Delta to see about changing our return flight tickets. The change fee was $100 but the new ticket would have been $800 more. (That included what we already paid) That would have been a total of $1800 for the two of us. The original cost of the tickets was $800.
Needless to say: We went home.