I have the trial for 9.5, i opens my 8 mixes ,and has the same plugins I use for audio recording, mixing, mastering ( Brick wall limiter, Room works reverb, exciter, studio EQ) 
Has anybody else played mixes they have created in 8 on 9.5, and found any difference in the plug ins
It is possible one of the plugins was not included in 9.5 , from the 8 era , yet i am not sure , i haven’t spent much time doing a side by side comparison . 
From what i can tell , the Brick wall limiter , uses a slide fader, instead of the knobs, that always used to stick, in 8
also , to make the VST rack , disappear in the event window , you click a mini event window screen just above the VST rack, insted of going to the left hand side and opening a menu
It seems the programmers at Steinberg, get a lot of feedback, and do quite a good job of streamlining the things necessary to speed workflow .
I just question , if there is a Plug in , like Studio EQ, that was dropped from the list . I used studio EQ as a PREAMP for instruments recorded at a lower volume , i could take a Bass guitar recorded low without any type of clipping , and add 12 db and get a really excellent warm big sound that way . Almost like using a Neve Preamp POST .
I don’t find any distortion in that EQ preamp , I stumbled upon it , and it has done wonders 
Cubase 9.5 is 100% compatible with Cubase 8 projects. All Cubase 8 effects and instruments are included. Anything that got new features or an updated interface is backwards compatible with the old version.