As long as you do not delete history you can go back to the unsmoothed version of your model.

After applying a smooth, you will see a smooth node (polySmootFace)in the channel box. Setting divisions back to 0 will effectively unsmooth the object. Be very mindful not to inadvertently delete history on your smoothed object as once you do this you cannot get back. Also, modifying the geometry of the smoothed mesh can cause issues if you try to revert back.