The last time I did this with Delta, they credited the difference in fare, but then charged me a $200 change fee. You need to talk with them first, but I would guess that they'd credit the $100 fare difference, but then charge you $200 to do it. They might let you use the $100 credit towards the change fee, but you'd have to pay the additional $100. ALSO
when I attempted to make a change, they said something about the credit only being available for airfare for the same person named on the ticket you are refunding, and it couldn't be applied to things like baggage fees, or be applied to another person, and the credit needed to be used within a year.