Can you use casing molding for baseboard molding?
As others have pointed out, it really is a style choice and nothing prevents you from switching the two.  But one thing not pointed out and you probably didn't notice, is the edge that meets the floor.  For baseboards, the bottom edge is square, where casing typically round the edges (which will not make as nice of a transition from the molding to the floor if installing over wood or tile).