I've done this (by hand! I don't even own a sewing machine.) on a cotton dress and it worked out well. I haven't done it on a formal item, so I would say you need to check if the dress has lining, boning, or other things that might make it more difficult. The dress I cut up did have a lining but only in the skirt portion so it wasn't an issue. The other main issue may be the zipper. It will probably have to be shortened or replaced if it goes through the bodice and waist. (Not a big deal, but it's good to know about.) I cut through the zipper on mine and it still works fine.
ETA: So basically I'd remove the bodice part, then take the existing waist band and fold it down however low you want it. Depending on how much fabric there is you might have to cut some of the waistband off.
And by the way, if I could do it that means it's probably a pretty easy alteration! I am by no means skilled.