Somebody who knows how to sew could probably fix it by shortening the crotch part, seeming the rim, relocating the buttons.
You might bring it to an alteration shop, that's what they are for. Although at the moment they are likely all shut down.
Have you thought about folding it to the desired length and placing a pin or two on it? Or magnetic pins if you don't want to actually perforate the onesie... This way you dont have to sew and you don't have to modify it/move the buttons...
your best bet is to find someone who does alterations but this will probably involve you haveing to wear it while they pin ot or chalk it, which would also mean wearing your nappy and waterproof pants if you use them. The other option is to have them custom made. I am UK based and use Cozy n Dry for mine in both nanyish onesies and plain coloured onesies (I have to wear 24/7 due to bladder incontinence) you can see a sample of Margarets work and a sizing guide at:- http://cosyndry.com/products/toy-time-onesie