With a couple little snips, I detached the black slip/underdress from the outer dress.
My plan is to flip the dress upside-down, so the skirt becomes the top. I trimmed off the top portion in a curve to give a shirt-tail kind of hemline. After a new hem, the bottom edge will be all set.
Now for the top (previously the skirt). I snipped the side seams down about 6-8 inches from the edge. Once hemmed, these will be my new armholes.
Then I repaired the ripped back seam.
Finally, I gathered the fabric around the neckline with a half-dozen small pleats in the front and back and joined the corners to form the new shoulder seams.
With my sheer shirt all set, I turned my attention back to the underdress. All I did was cut off the bottom half and hem the edge. Now I've got a black cami to be worn under my new top.
And to put the two back together...