I had both Keyshot 7 and Keyshot 8 on my computer because Rhino plugin was not supported. After deleting Keyshot 7 ksp file format default program is unknown application. Also before deleting Keyshot 7 I tried changing default application to Keyshot 8 but when I choose Keyshot 8 again it opened via Keyshot 7. I tried change it but it did not work.
Win 10
KeyShot 8 Educational
https://support.microsoft.com/en-us/help/4028161/windows-10-change-default-programs
I tried that but it didn't work then I reinstalled the program it solved.
I am actually experiencing this as well on both my work PC and home PC.
I have narrowed it down to Windows 10 ver 1803. It won't save my changes to ANY of the default app modifications I make. For instance, I am unable to make .jpegs open in Photoshop instead of Windows Viewer.
I have tried uninstalling and reinstalling programs, and even had to create a new user profile and switch everything over to that to get it to stop opening Ks7 automatically.
I had the same issue after KS7 and KS8 updates. The only way around was to delete everything and clean the Registry for anything that was KeyShot related. Then reinstall everything and it works.
My KS files would default to which ever app I installed last. Discovered this when Speedster had the same issues with the beta.
Creating a new windows user profile did the trick for me (effectively a new registry). But as I said, I think the root of the problem lies within Windows 10 1803 update. Even though, I have no problems with opening Ks8 docs now, I cannot change other default apps, such as the .jpg example I gave earlier.
It just became apparent to me because KS8 was just released, thus, the need to change a default program, but the blame in this case goes to Microsoft for failing to solve this issue. I can't even roll back to a previous update because they've removed that option in the control panel, and I'm too dumb to find a work around ::)