Step 1
Start from back.
First draw the new hemline, then erase the dart and replace it (see the red lines):
- with a princess cut ending at armhole absorbing 1-1,5cm, lines overlapping by 2-3cm at hem to widen it slightly
- shaping center back by 1,5-2cm at waistline.
Draw the upper back detail as marked from the light blue lines.