How do you guys maintain old projects with old plugins? Is it enough to just keep all the files in the VSTplugins directory?
 
The context of my question is what if you had to re-install from scratch on a new system, do you have to start from your oldest version and keep installing so you get each plugin reinstalled? 
 
OR
 
Can you just keep a backup of all the plugins in the plugins directories and restore those after you install only Platinum?
 
Thanks

Both, Some plug ins are built into Sonar and cannot be installed independently.  I always had to install 8.5 to get True Piano's. 
There is a thread just the other day with a list of Sonar versions and plug ins. 
But for me if a plug in is missing I find using one of the newer ones is an improvment and I don't fuss with any 32 bit stuff anymore. And I tend to finish everything I started anyways for this very reason. 
But sometimes I like to fiddle with a re mix of an old song and that's when I would rather just use the new tools anyways. 
If there was some trademark sound from a synth then you should have froze the track way back when to make it future proof. 

Will it also cause a problem if I uninstall old versions? Like X1, X2, X3?

If reinstalling from scratch on a new system, a disk image may be the best route to choose.
 
I just reloaded this machine from scratch recently, and reinstalled X1->Platinum. From a straight-up size perspective, each version is only about 200 MB, so is not a great benefit to remove them. Third party plugs are more the PITA to reinstall (some would work via copy/paste, others require installation/registration depending). As Johnny mentioned, if I open a project from eons ago, I will typically immediately save as with a new name to include the version and if plugs are missing, I will typically replace it with a current 64-bit version of the same type.
 
I am leery to uninstall an older version, as I have not seen the uninstaller run but think it would honor newer versions of the same plug, or ones potentially used by other (newer) versions. Just in case, I would probably re-install Platinum again just to be sure current plugins were intact. I "assume" the uninstaller would also allow for tactical removal, but again never used this.

There are few plugins that are actually tied to a specific version and the core Sonar program.

For instance if you uninstalled X1 you wouldn't lose the Sonius fx suite as long as you elected to not uninstall associated files.

There's a spreadsheet of plugins that come with versions 8 to X3 available on my Onedrive. See my signature.

Thanks everyone!
 
mudgel, I will check out the plugins list