Flights arrive early and late all of the time, this is why the rental companies ask for a flight number on the reservation.
First I would check if your flight number is still the same (or if it has been changed). If it's the same as the one on your reservation you should be ok or at least have something in writing that you can present to the rental agent to support your case that the airline had brought you in early.
Second, an exact 24 hour rental time (2PM-2PM) is usually more expensive than a slightly less than 24 hour time (ie: 2:30-1:30). I would try to see if the rental cost would be similar to your original price if you had slightly better times. Don't forget, it will take you some time to deplane and pickup your luggage so this time can be added to arrival time. For the drop off time, you will want to put something in the area or 1-2 hours before flight time (more if it's an international flight.
Third, I think I'd check on the Transportation forum here and gather a few codes and see if you could get a similar rate with a new reservation from one of the other companies (then just cancel your original reservation). Our personal favorite is
Alamo for the ease of rental, but have had good luck/prices with National/
Enterprise,
Budget/
Avis,Thrifty/Dollar, Hertz. All are onsite and with the right combination of contract IDs and codes (found on these forums), good prices can usually be found at some if not all.
Now I cant speak for 7 hours early and Dollar's policy, but we had a plane/route change on our flight last September and arrived almost 4 hours early.
I tried to change the arrival time online but since it was the day of the rental, the price was almost triple.
We decided to just take our chances and try to pick up the vehicle when we arrived (4 hours early).
The clerk never even batted an eye at the early arrival and the price on the receipt was exactly what my reservation said.
As I said, our flight was not as early as yours and we were using Alamo (not Dollar) so our experience may not be totally relevant, but know that Dollar WANTS to rent you a vehicle, not to try to find a way out of it.
It's definitely preferable for you to show up early than for you to not show up at all.
The only problem I could foresee with showing up too early is if you have a vehicle booked that has limited inventory (like a convertible or sports vehicle)