Sara had a dress that she wanted to be a skirt and I agreed to try. The first snag I ran into was that while the skirt was cotton, the top was stretchier. I chopped off a sleeve and tested it - seemed to sew ok.
Then I cut off the bodice
I tacked the top to keep it from fraying then folded it over and sewed it along the seam
A little tricky, but I think it worked
There's a space left to put in an elastic waistband and/or drawstring as desired.