It is not a hassle at all to switch rooms, assuming they have something else available. Even if you do online check-in and your packet is already pepared, you can ask to switch if you don't like the room or area. If they are not full, they will work to accommodate you. Last year, we were assigned Casitas 2, which had not been renovated at the time, and I really wanted to be closer to the Dig Site. They moved us to 9A, which was great.
If you are sure you don't want the Ranchos, you can call and add it to your room request. You can also send a fax a few days before you arrive.
We were in 8A this summer and it was a great location, but I think Ranchos 7B might have been a little better. We were close to the Dig Site and the cabanas bus stop, but it was a hike to the laundry - we were in between the cabanas laundry and the ranchos laundry. I walked over to the ranchos laundry and thought 7B would be a great location - close to the Dig Site and also close to the laundry and quiet pool. We don't spend that much time at El Centro, though. Being close to the pool and bus stop is more important to us.