So for those interested, I have not had a chance to try to fix this on my main machine, but did get the drivers and new interface working on a laptop.  There I have no problems opening my old audio files.  So, I think I'm left with a clean Windows install, and possibly a dual boot as well to keep a DAW-only clean install.
 

A lot of these weird issues are very system specific. 
There is a million ways to build and configure a PC. 
You might re install windows only to find the issue didn't go away because it's in the BIOS. 
That is why Macs seems to work better with audio applications. 

Well, I did have some luck today.  I installed a separate instance of Windows 10 on the same machine (for dual boot).  This way I could see if it worked on a clean system without committing to a full reinstallation of everything. Sure enough, it works just as expected.  When I open a file mapped to the old interface, I get the remapping dialog and after that it works fine.  So after all it does seem like some problem with my installation of Windows (and software, drivers, etc), and not Sonar.

Thanks for retuning to update. It makes these threads educational for others.