In that case, the would have your device looked at ubreakit or a Samsung experience shop.
There isn't a "certified public" way to downgrade it. I can bet you all the money in the world that techs in the Samsung R&D department do it on a regular basis. They just don't want us to do it.
Sad thing is, colorless flat ui with tons if blinding white and preschool looking icons is the in thing and it doesn't appear to look like it will ever go away. The only true downgrade path is via an older Samsung Galaxy not supported by this new update. Like an S7, S6, S5 and so on.
I hate that I've been dealing with all of these new "improvements" for the past week and a half. One of the several things that I hate about this update is that the new Message+ icon no longer shows that there are unread text messages. I'm missing them left and right, and this phone is essential for my work. I'm also constantly having to restart my phone to get certain apps to work correctly. Again, I need these for work.
If thise update doesn't get resolved soon, I'm looking for another manufacturer who won't mess me/us over like this. Bad form.