It's wedding season! Here is one of Valentine's flower girl dresses and this is how I let it out.
Start with the zipper open, and the dress inside out. The center back seam in the lining has a top stitched section. This is the gateway to the innards of the dress. The lining is sewn into the hem; otherwise this would be much easier.
Pull the inside of the dress through the hole in the center back seam. Have a mild heart attack, and then find the tail of the zipper.
Follow the tail up into the center back seam. There should be four intersecting seams but only two are sewn to the zipper. Just spread the seam allowances and check for the edge of the zipper tape. Gotcha!
Follow that seam, working your way to the neckline, turning that side completely inside out. You will likely need to clip a few tacks.
On the tooth side of the zipper, pin baste the lining to the outside of the dress. This will make sure they stay lined up when the seams are picked. I let it out from ~1.5" inches below the neckline to ~2" below the waistline.
One side of the seam will have two rows of stitching close together. The other side only has one. Pick the outer most seam and tie off the loose ends ~ 1" from the neckline.
Work your way down to ~2" below the waistline. Tie off the loose ends. You may encounter some difficulty in the area of the waistband; work gently with the seam ripper. The waistband and bow may come off with this seam; we'll put them back on when we are done.
Now this is what it should look like. The lining and outside of the dress are separated. The zipper is still attached to the outside of the dress.
Pick the remaining seam and tie off the loose ends ~.5" before the previous seam was tied off. Work your way down and do the same at the bottom.
Everything is separated!
Pin baste the zipper to the right side of the dress, gradually moving it towards the edge of the fabric. Don't let the edge of the zipper tape go beyond the edge of the fabric. Use a zipper foot to sew close to the teeth.
Sandwich the zipper between the lining and the outside of the dress. Thread baste through all layers, 3/8" from the zipper seam, on the tape side. Here you can see the new zipper seam (in black) and the thread basting (in white). Sew through all layers, 2/8" from the zipper seam on the tape side. Remove basting stitches.
Repeat on the other side.
Turn the dress through the hole in the center back seam. And make sure everything is okay before sewing shut.
If the waistband has come loose, hand sew to the original seam line, using a blind stitch. And hand sew the bow back onto dress with a few tacks.