As it turned out the shortest shelf just happened to be the one my old desktop monitor needed to sit on. Though it did fit, my fear was that it would eventually bow the shelf it sat on. This weekend, while my hubby was home, I asked him to help me change the brackets from the bottom of the shelf to the top.
To make sure the brackets were in the same place, I had hubby drill, using a very small drill bit, clear through the outside holes for each bracket. The middle holes were not centered so we did a quick predrill after the two outside screws were in place from the top side of the shelf.