We need more information.
From your description, you are about to have a Le Fort I maxillary shortening / impaction. In this procedure, a horizontal sliver of the mid facial bone (maxilla) is removed and the lower segment (containing the roof of the mouth and teeth) is moved up / down / backward / forward, as needed (usually to correct the way teeth touch (IE occlusion).
In many cosmetic cases, this USED to be done when people had a Gummy Smile and showed more than just tooth white when they smiled - you saw their gums. It was treated by maxillary shortening - the operation you are having.
These days, this is largely fixable with well placed Botox. (Since the facial soft tissues stay the same, shortening the bone height results in the upper lip hanging lower and covering the teeth).
I cannot understand WHY in your case the maxillary height cannot be maintained to allow for visualization of the teeth.
I would advise you to get at least another opinion from an experienced orthognathic surgeon.