I asked about this on my first trip because I booked a 1 bedroom and decided I needed a 2 bedroom a few days into the trip. (okay, I didn't need it, I wanted it! I didn't want me kids in my living room) He told me I could upgrade using additional points, I could not use cash to upgrade. He did tell me I could use up the rest of my points that I had on the reservation for a 2 bedroom and then pay cash for the remaining full days. But this would probably not work if you were renting because the number of points would not be an exact number for X number of nights in a 1 bedroom vs X number of nights in a studio. I could either borrow or bank the remaining needed number of points.