"updowngrade", funnily enough, I would understand as having the (IMO misunderstood/wrong) meaning you're trying to give to "Verschlimmbesserung" in your post here https://news.ycombinator.com/item?id=49483585 (making some parts better and some other parts worse.)

That's not what "worseprovement" conveys to me, you can worseprove a single part, intending to make it better, making that same thing actually worse by the very same change.

It's close enough and has been already in use. A regression of features quality - for example.

There's no reason to coin a new term at all, nor chasing this idea of using long German word as a replacement.