We were forced (due to weather delay) to pick up a vehicle 16 hrs late. We called Hertz when we found we were stranded, and had no problem making the pick up later. When we got home we had the change and new reservation confirmation waiting for us. There was no change in the bargain rate we had. You might consider waiting until the last minute and requesting a modification, however, if it doesn't work, then you would be stranded w/o transportation