When I have made reservations in the past, particularly at AKL since the view is coveted by many, the CM would say that the rate guaranteed a particular view as a minimum. In otherwords, a pool view rate guaranteed a pool view or better, i.e. you might be upgraded to a Savannah view, but would not be down graded to a standard view facing the bus stop or parking lot.
If you want a guarantee of a particular room group, you have to make that kind of reservation, but there is a good chance for an upgrade at check-in during slower times of the year.
Also, another way around the upgrade question is to find out from other Disers where the type of room you want is located and request that type of room. For example, all of the bunk-bed rooms on the Zebra trail also have pool views and a few have Savannah views. I reserved a standard room and 5 days out I faxed a room request to AKL and requested bunk beds on the Zebra trail. We were upgraded to a pool view which we prefer to a Savannah view. If we had preferred a Savannah view, we would have reserved a pool view and requested the upgrade to a Savannah view.
However, I think Disney has begun to discourage contacting the resort directly and prefer that you submit your request to the CM at the CRO when placing your reservation. If there is an additional charge, they will tell you, but if they don't charge extra, they can't guarantee the upgrade if there is a difference in cost between the room group you pay for and the upgrade you request. In your case, with no additional charge for a king bed, you will probably get it.