If you want to open all Sonar files with Platinum, right click on any of the old Sonar files and select choose default program- open with... SONARPLT.exe
And check Always use the selected program to open this kind of file.

On a side note, I once had a dream that every Sonar file I clicked on would open in Magix. I still get chills down my spine when I think about it.

Is it possible that you may have done a custom install of 8 . 5 to grab a couple of plugs or soft synths that don't come in SPlat ?
 
Kenny

Very good point Kenny. This is distinctly possible and would definitely go under my radar!
 
Things like V-Vocal & Pantheon spring to mind

I'm having what he is having!

JustGotPaid
...I can't find anything about 8.5 on the machine. I've done searches, I've looked through program files, I've looked through old folders with earlier Sonar recordings, I've look in the trash bin...I've look every where I know to look, and I can't see a trace of 8.5...

 
Firstly I would look under Installed Applications in Control Panel (or the W10 equivalent, whatever that is). I would also be curious to know what options you see when you right-click on a project file in Windows Explorer and "Open As".