I uncharacteristically forgot to take a before photo of the dress, but this was the bodice, and the skirt started straight under the velvet bit (the bit of silk between the bottom of the bodice and the waist has been turned into a scrunchie for my hair - waste not, want not, etc. The velvet won't be thrown away either.)
And this is the finished object. It's fairly full at the bottom.
Close up of the back of the waistband. It could be neater, I didn't leave quite enough ribbon to overlap properly, and the two halves aren't exactly the same height. The zip is very short because it's the original zip from the dress, and that's where it came to. I did contemplate taking it out and re-inserting it further down, but quickly gave up on that idea. If I was doing it from scratch I'd have used a concealed zip, but this one will have to do. The fastening is a hook and bar, buttonholes are beyond me.