Search the tutorial forums. In short create new actors to link models to units. Under the unit put the ability in the field under Ability - Abilities - Ability then designate a button and command card slot for it under the field just below.
As for the hiding the transition you could use the actor event Set Opacity to blend the old unit out and the new unit in. You could do like the zerg morphs and use a dirty great big cocoons or you could use the Creep Spore Emmiter model to create a cloud that hides the transition.