Your description is a little vague. As Steve said, you can either add another 1/2" of plywood to the lower floor (glued and screwed or ringnailed to the joists) or you can just dress up the transition. What concerns me is what you might mean by ..."and then continues downward slope"... If this means your floor looks like a roller coaster then you might be better off addressing the structural problems in the floor before redoing the finish above. Otherwise it would be like re-painting a car before you fix the dents. If this is the case, maybe send us more info on EXACTLY what is going on.