It's going to be hard to upgrade classes in the middle of your rental. You might be able to go back to the original renting location, but I think they will struggle with charging you an upgrade fee for only a portion of the rental.
As far as an oil change goes, often you can get it done on your own at a local garage and Hertz will reimburse you, but you'd need to call first to get authorization for that after letting them know the mileage. Alternatively, you can try to go to the location and swap cars for something equivalent, but again, I'd call them first to clear that.