I rented a van with a drop off date but am might need to drop off at a different airport. Is this possible?

Yes, it's likely that you CAN, but it's even more likely there will be a significant "drop fee". Call the rental agency for the details.

I don't know about changing it after you've picked up a car, but I have made a reservation where I picked up at one airport and dropped off at another. So it's worth asking.