If you didn't pre-pay, keep your original rental, and make a brand new rental for your new plans. Keep checking a couple times a week as prices fluctuate often. I recently booked into DAL. When I originally booked, prices were over $400 for a full size on my dates. After a couple weeks I was able to book at $148. Now it's back around $325.
Just keep trying, and make sure to search all brands.