I had previously tried that, so let me describe what happens: When clicking on the .cpr file, the Cubase 9.5 installer fires up if Windows cannot find the 9.5 executable.
On Windows 10 starting with 1703 I believe, Application associations are handled by a new XML-based system. The problem could be as simple as two lines being swapped in that XML. Steinberg would supply the XML associated with their associations as part of the Cubase 10 install, but I cannot locate it, so maybe it’s missing.
I have the same issue coming from Cubase 7.5. Neither method 1 or 2 in the first post are working. Windows will not allow me to set Cubase 10 as the default app for CPR files. No matter which method I try, the screen ‘blinks’ once but then Cubase 7 remains as the default app.
I usually open projects from Cubase, and eventually I save as… if you need to keep the old project name the new adding 10 or whatever you prefer, otherwise just save, it will ask to overwrite the old and it will open Cubase 10 from there on.
In my case all .cpr files open with an older version of cubase (C7.5) regardless of which version I use to save the project. Even a new project created with Cubase 10 opens by default with the older version of Cubase.
I’m using Windows Home 10 - V1803 - OS Build 17134.407