Michigan's system stops that kind of stuff: if you make a reservation, and then want to change it - you lose the entire reservation, and have to start anew. Also - you can't reserve a spot starting on a Wednesday (but you really intend to show up on Friday night) - you HAVE to occupy your space within 24 hrs.
On the other hand, I've had luck in scoring spots at the popular parks by simply checking on a regular basis, for folks who've had to cancel their reservations for whatever reasons. But in spite of all that - the popular parks still fill up within hours of their first "availability to reserve" 6 months ahead of time...