On a copy of your basic block, first draw the new hemline above the knee. Then taper 2cm inwards from the side seams, blending to hipline.
Shape center back for the dress better to follow the back curve (I would suggest 1,5cm at waist but it depends on your body).
Draw the new neckline with a soft V on both back and front. On front, widen shoulder dart by 1cm at neckline as we usually to on very open neckline.