I don't know for sure that I do have to use it for international. Years ago I had to cancel an international flight and apparently was only supposed to be able to use the credit on another international flight. Since the phone agent didn't tell me this I was able to get Delta to let me use the credit for domestic but have assumed that I would have the same type of restrictions on this.
My concern is that Delta seems to treat Mexico/Caribbean and "semi-international"
Since I don't know for sure if I need to cancel and Delta is still digging out of last weeks mess I didn't want to tie up the phone lines with a "what if" question!