It really does depend on how the dress is made.  I am working on a dress right now that has no seam around the middle but it has a band at the bottom of the "empire" waistline.  I am removing the band and making a seam to make the waist shorter.  I have worked as a bridal seamstress for 13 years and have sewn for 27.  I really would depend on how it was made and a professional seamstress should be able to figure it out.  Sometimes when there is now way to shorten the bodice above the waist you might have to let out the hips to let it drop.  Hope this helps!