Start from back. Discard the dart dividing it from center and side, blending the new lines to hip level. Draw the new neckline from armpit to center back, perpendicular to it in the last few cm.
On front, work on half first and draw the new neckline with a soft heart shape, then widen the shoulder dart by 1cm at neckline to have the dress tighter to the chest. Lastly, trace a copy of full front.
On both front and back draw the spaghetti strap sitting halfway on the shoulder line.