You can change your reservation for a non-refundable fee of $10. Call the number on your reservation and they can split your days for you and give you a refund for days you are not using. I called because I have a week in Wawona and I might need to change it to only 4 nights or two at the beginning and two at the end (around our valley stay). They told me that they can adjust a campground reservation any way you like, for a $10 fee, but to make sure you call at least two days ahead to avoid any other fees.
Empty sites bring lots of questions to campers around those sites and I have seen people scoping out sites for later use (they come in at night, set up, and are gone by morning). I don't know if they have a reservation or not, but once the host in the kiosks are gone by 3 or 4, anyone can cruise through a campground. Since the reservation is in your name, you might be liable for anything that happens while you are gone (bear box citations, etc.). Better to be safe than sorry!