I have many Sony Vegas projects that I created using various Waves NPP 2.x plug-ins.  I have since upgraded to Waves NPP v3.51 (aka 3.6).  When I open Vegas projects that used the 2.x versions I get an error saying that the plug-ins are not installed.  The Vegas projects open, but without loading any of the Waves plug-ins that I used.  All my plug-in chains and settings are lost.
Why is this?  As far as I know, the names of the Waves plug-ins are the same in 2.x and 3.x.  Is this a problem with the way Vegas refers to plug-ins, or is this just the nature of plug-ins in general?
Obviously this is a major hastle when one has to re-work an old project.  The only way I see to fix the problem is to remove the new Waves plug-ins and re-install the old ones.  (Although I am not sure if this will work.)