I don't see why the OP didn't change his reservations rather than just cancelling. When I have had to "cancel" my GA state park reservations, I go ahead and look into my calendar and simply change my reservations to a different date that I should be available. That being said, I rarely make reservations way ahead, because I know my schedule often changes.
What I hate is paying a Reserve America fee to check into a SC state park for an overnight stop when traveling. It really bothered me, when I got to state park after office closed and had to wait on them to open to leave so I could pay for the previous night. Yes, just to access the system for my registration, I had to pay a RA fee in addition to advertised camping rate. At least, GA just includes any cost of using RA in their rates.