I have reinstalled Sonar and the VSTs several times over the years (PC and HD upgrades, etc.)
When I open old projects I get the "The following plug-ins are referred to by the project but can't be found" error, even after scanning.
Usually it's DimPro.  I can select then 'replace synth' with DimPro but I loose my program and don't remember what was there years ago.
Even when I put a program in, it won't work even after setting the I/O to and from the synth.  I have to insert a new instance then relink the MIDI tracks.
Is there an easy workaround?  I plan to install an SSD soon.

The problem with DimPro is probably the format change of the plug-in from DXi to VSTi. SONAR does not recognize the DimPro installed on your machine is now a VSTi. I believe it is possible to have bot the v1.2 DXi and v1,5 VST installed at the same time. There is no way that I know of to tell SONAR to automatically use a new VST plug-in for an old DX one.

You're probably right.  I think the only way to open the original to see what program was used is to reinstall the DXi (I wonder where I find it?)

It will be on disk from either a previous version of SONAR (it was included in several versions, it has it's own installer on a separate disk in the SONAR set) or the original DimPro disk. Then patch the DX install to v1.2 using the patch from the DimPro page. You might have to uninstall v1.5 before starting. You also will not need to install v1.5 from X1 or X2, the v1.5 patch from the DimPrio support page is all you need. It may be necessary to copy off the old DXi before applying the v1.5 patch, I do not remember if the v1.5 patch deletes the DXi. I just put up the the format change.

1.5 is VSTi only whereas 1.2 (and earlier) includes both VSTi and DXi. I believe that installing 1.5 will delete the DXi.
 
If you can't find your copy of 1.2, it can be downloaded here:
http://www.cakewalk.com/S...kb/reader.aspx/2005302