What happens if I uninstall 8.5 Pro now that I have 9 working well.
Can I simply run the 8.5 Pro uninstaller?
Will it mess up 9 Pro, by way of removing anything I may need? I realise all my own VST installs are independent, but I wouldn’t want it breaking any core functionality in 9 by removing any core shared system files.
Need to be sure about this, so please don’t guess the answer 
I can report that I have removed several previous Cubase versions after upgrading without any issues. I uninstalled them from the Windows App/Features menu.
So I am sure that I had no issues removing old versions and I have seen many other users remove theirs too without issue. 
Regards 
I think Prock has given as good an answer as you can honestly get.
There should be no issue and I too have removed previous versions without ever having problems but I don’t think anyone can absolutely guarantee that you won’t have one.
That’s great. Cheers!