Be sure to sign up for Dollar's Fastlane. It will save you bunches of times and they often offer you better vehicles for the same price.
Even if you have an existing reservation, you can call Dollar and they will transfer your current ressie (with any discounts) under your Fastlane number. Don't cancel and make new, just for Fastlane. Let them transfer for you.
I just changed my Spring Break ressie from KISS3 ($229/wk) to EMN39 (#199.99/wk) Thanks for the info!