Be very careful with this! I tried to modify a National reservation online last year and it was a huge mess. I had booked a minivan at a great rate months before our trip. The airline canceled our original flights and we had to arrive 6 hours earlier. I tried rebooking online, but the prices were double. Then I called and was told that any change to the original reservation would result in having to rebook at the current rate. So I tried the "modify reservation" approach. I plugged in the new arrival time and the rate was the same great rate. So I printed the reservation. But when I went back to check the reservation online, it was changed to the higher price. It took me at least 6 phone calls, and numerous emails, scanning the printed confirmation, etc. to get it "resolved." In the end, I had to speak to someone in the corporate office, who did something to override the computer system. She could not offer the reservation at the same price, but offered it to me for $34 more. She could not send me a written confirmation of the reservation with the accurate price, and the online reservation number showed the higher price. I printed out all correspondence and brought it all to the airport. Thankfully it was not an issue, we got the van at $34 over the original rate, but it was an extreme hassle and very stressful. And this was just to change the reservation by 6 hours, not even an additional day.
I hope it works out better for you than it did for me! Good luck!