Slimer first discovers that Victor is a shapeshifter when Victor transforms from the small, cute ghost into a large and muscular ghost in order to break open a door. Victor would not have had to break down the door in order to enter the room. Ghosts in the series have the ability to travel through doors and walls; Victor himself demonstrates this when he passes through Ecto-1 without being hindered. In summation, it was unnecessary for Victor to shapeshift in order to pass through the door.