I ran SONAR 6 PE right up until 2013 ..
What that means is I have a ton of my old SONAR 5 and 6 PE projects on an external HD ..
 
On one of my computers I made sure to install the plugs I wanted from back then to have them in X3 PE & Splat..
on the other computer  I skipped out on it ..
 
Aside from a warning box indicating a missing plug or synth , the only Wonkyness I have experienced so far while opening some older projects from the SONAR 5 and 6 era has been , when I opened them up in  SPlat , I had to go into my console view and tidy up a few things . I use a 2i4 currently. Back then I used a Delta 1010lt and a Audiophile 192 combined together.
IIRC, all those outputs still showed up in my console view in Splat when opening up those older SONAR projects...
 
all the best ,
 
Kenny
 
 

I save all my songs as MIDI files because that's where a lot of work goes. My files from my Atari 1984 still open just fine.   Audio files are audio files so will open too but then I can always play a part better, sing better etc. And as far as effects go why would I use an old outdated ( 32bit) plug when I now have my nice selection of up to date plugs that all do the same thing but much better. 
I have opened  CWP created with  Guitar Studio or 7 or 8.5 and they always load but with a list of missing plug ins, they even show in the bins but grayed out, so easy to replace. What's the big deal.  
I've never had a missing VST because I don't use anything strange and I still have 8.5 installed just for that one reason alone. 
 
I can see it's not good losing a synth sound that was a part of the song but the solution would be you should have frozen it if it was that important. 

My only problem with old projects is due to me having used the DXi version of each softsynth for which there was both a DXi and VSTi version. The DXi versions have subsequently been discontinued and therefore excluded from the upgraded versions of the synths. This applies to DimPro, Rapture, Z3ta+, Battery3, SampleTank and Miroslav. For example, the DXi version of DimPro exists for version 1.2 but not for 1.5 and it is not possible to install DXi 1.2 and VSTi 1.5 on the same platform.