A new installation of Sonar Platinum is reporting missing VST plugins.  
 
Note that I usually keep all my VST plugins in one folder and have pointed Sonar to that accordingly.
Note also that I use only 64-bit plugins on new projects, and the project in question is only 2 weeks old.
 
The confusing thing is that Sonar sees the plugins in the plugin browser, but it is not able to reconcile those with the existing project plugins.
 
Is there any way to either:
(a) coerce the project into replacing the plugins (without losing their settings); or
(b) save the plugin settings so I can reassign them?

Check the search paths in Plugin Manager. You probably had some plugins installed in locations outside the default locations, and the re-install re-initialized those pathnames.
 

Thanks bitflipper, but I only have one location for my VST files and I was pointing to that.  What I did find is that the VST scanner utility was sitting in my task manager "doing nothing" (presumably stuck?)  I killed it and started a new scan and that helped a bit.  
 
What helped even more was to go through the installation process of some of those plugins (from Izotope and Cakewalk) and target the same folder.  I suspect -- and I could be going out on a limb here -- that there are some registry settings or something else that I am not seeing.  All the other plugins are just fine

To create a scan log by hold the SHIFT key when starting the scan. There are times, the scanner throws up an interactive dialog which might be hidden behind another window. Also when the scanner hangs on a plug-in, the TOAST message shows the problem plug-in. Moving the plug-in out of the scan path, re-scanning to completion, adding the plug-in back and re-scanning often puts everything back. A reset might help but often hangs on the same plug-in.

Thanks scook, it's a good thing to know about the log file option!

So I eventually worked through this issue by reinstalling the missing plugins from an executable.  I had wrongly assumed that the DLL was sufficient (well, it had worked for me that way before).  Now things are not so simple as a DLL.  I kind of miss the days when installing any software meant copying a folder.  In today's world, a simple update of your system hard drive means you can pay dearly in time and effort