You can call the day of and check about extending it. It might re-price, though. My guess is that you'll have to trek back to the rental car center.

With Avis, I've had this many times and only once have I been able to call and them check-in the car and check it out again. It's against the policy (what if you damaged the car and had insurance on the 2nd rental, etc), but it was a local station and I knew them well.