When using the Sub-D tools, there are always two... the cage, or the facetted object that establishes the structure of the sub-divided version, and the sub-divided version itself over top of the cage. So, your model is always facetted underneath, but looks subdivided, and as a result "smoothed". You can toggle this state by using the Subdivision Swap tool. Make your facetted edits, and then switch back. In addition, you can use tools that work on facetted objects to change the shape of the sub-divided model even when it is in the sub-divided state. The caveat is that it should be expected to work on the facetted model as if the sub-d was not applied. So if you know well your form.Z, you will be able to predict what will work and what won't. It is best to keep four-sided faces for sub-division surfaces, otherwise the results can become unexpected.
Hope it helps.