I will only give my personal point of view.
I don't find it necessary to upgrade Toxicroak and Seismitoad to OU.
The use of a pokemon in a tier depending on many factors, I don't believe it makes sense to only consider the percentage of use of these.
Currently, for sure, if there's one Pokemon to raise from UU to OU, it's Porygon2.
Regarding the pokemons that go down from OU to UU, I think the list is correct even if I have a slight doubt on Salamence. Also, I think it would be nice to give Haxorus a try in UU.
Regarding pokemon going down to NU, I find these changes interesting.
Defensive teams will recover some support, which will probably allow us to see some more diverse teams than at present.