I'm sure this has been discussed before, but I have a 30/70 chance of having crazy crashes and/or plugin problems when opening projects that are pre-Platinum (i.e. X2a and X3e) in Platinum.
 
• I just had a crazy crash where Vintage Verb "lost" its license key, the project came up blank (i.e. no tracks), and when closed/quit, the Sonar main window closed but kept a fast-flashing phantom window over the desktop with the CPU pegged at 100%.  WTH?
 
• Other projects from X2 and X3 can have nasty lockups caused by 32/64-bit plugins used over the years.  It seems, however, that iZotope (64-bit only) plugins don't play nice between Sonar versions, as I sometimes will get Alloy 2 crashes.
 
• Another problem I've experienced is sporadic behavior when moving clips in the Track View.  Creating a new project and copying the clips over from the old project to the new project alleviates the problem.
 
• Another is VST3 botch-ups.  I experienced some crazy MEGA-lag issue with a Waldorf Largo instance in a particular project.  It would load fine and play fine at first, but when would "lag out" big time, causing massive CPU spikes and delayed audio.  Even manipulating the control knobs was lagged.  Deleting the VST and inserting a new one would alleviate the issue.
 
I don't get it.  If anything, new versions of Sonar need to NOT introduce these kinds of incompatibilities because they are hard to troubleshoot and remedy.  I am too haphazard to confirm every single bug down to a single or multiple culprit(s).  My creativity is already going nowhere, so I can't waste my time troubleshooting incompatibilities.  If all I did was mix other people's audio, then I think I'd be in heaven.  But lately, trying to create music has just been a HUGE mess.

I've often said that the longer you work on projects in Sonar, the more chance they have of developing oddities and instability. I've had projects from X1 and X2 which have developed so much weird stuff that I've simply abandoned them and recreated them from scratch (which actually turned out for the best). However I still have a few old projects which are large and cumbersome which I tinker with from time to time but wouldn't want to start the mammoth task of rebuilding, and they can be a PITA stability wise.

I have 8.5.3 bundle files that open up in Platinum.  I sometimes have to disable old plugs that were not 64 bit ready.  I have not had a problem in this area and nothing that I would attribute to Cakewalk.

Hi. I have during the past month or so opened and worked with several projects that were created as early as in SONAR XL2.2, and have had no problems besides removing old (missing) 32 bit plugins. When I open projects that old, I remove every plugin / synth I no longer have or that are 32bit and save as new. And they work as they should... Sorry to not be of any help.
All the best.

The only projects I have any problems with have been unstable all the  way back to X1, so I don't blame Platinum per se, but the vast majority of projects open & play fine, missing plugins aside

I had two this week that I had not worked on in several months (big projects with huge track numbers) that Kontakt wouldn't work right in. Kontakt kept crashing the start up so that I had to open in safe mode and allow each instance of a synth to completely open before saying yes to the next. Then once the project finally opened none of the instances with Adagio Strings would work. Even the screen on Kontakt was all messed up for them. Then instances of Kontakt with instruments from other companies would play the wrong sounds! Strings were drum kits and so on! If I inserted a new instance everything worked fine but anything old was messed up. I am having to rebuild the thing from the ground up. I have never seen anything like this before this week.

I have seen the "VST un-registering" issue on a couple plugins, but not in this version. I am trying to remember if this was before or after I did a "restore image" or not (I think it may have been), and not sure if that played into the scenario.
 
I am not sure if this will help or has validity, but when bringing forward an older project the very first thing I will do (after acknowledging missing plugins and the project is open) is "save as..." with a new name for that version. After that I will close and reopen the file so that the current version is working with the file saved by that version (in hopes to bypass any possible cwp configuration tweaks). This may be overkill, but the "save as..." is definitely recommended so that the original cwp is left as it was (and be opened in the version it was created in just in case).

mettelus
I am not sure if this will help or has validity...This may be overkill, but the "save as..." is definitely recommended so that the original cwp is left as it was (and be opened in the version it was created in just in case).

 
This is SUPER valid and helpful advice. I've been doing what I do for a long time, and have gone through multiple versions of multiple types of software. As a result I've encountered what the OP has encountered in numerous programs, and your advice can solve many issues.

at the end of the day, it's just poorly coded software.   no matter how many times you save a file, it shouldn't increase the chance of corruption and/or instabilities.

Cakewalk have been building up from the same code for a very long time.   

Its been my experience that all versions of Sonar can and do run all projects without issue. Even when a plugin is missing Sonar notes the fact and runs the project just fine.  My projects go all the way back to Pro Audio 9. Not one of them has failed to open or run. The OS was Windows 95 back then. Nor have I suffered from file corruption.  
 
As far as CW using old code, saving and loading are very basic procedures with any software. Old or not it wont change much over the years.