"Merge" reservations?
 

I booked a rental for LAX that was supposed to be just one day, but is now going to be extended to two days. The per day rate for the car is $15 more expensive per day, but if I just keep my current rental, and book a second for the extra day, the rate stays the same. It's the same class, for both, will they allow me to just start the second contract w/o coming back in, or will I be forced to go back to LAX and start the new contract?