Still using the latest version of Sonar, before things tanked last year....but I did get Studio One and actually use it quite a bit as well.
 
With all that mentioned, I didn't ever open a Bandlab account, to my knowledge....maybe did.  But anyrate was under the impression the full version of SPlat was still better than the new free version....all my old plugs worked, etc, etc.  Therefore I never d/l the free Bandlab version.

At this point is it worth switching over with all the Bandlab updates, now released?  My year+ old Sonar still works fine, but probably should keep things current.  I don't especially want to loose any of the old SPlat functions, unless they've been replaced with new, upgraded and better features.... and again I guess I'm just talking about the plugins??  IOW, how does my "loaded" old SPlat compare with the current Bandlab version?

Keep SPLAT installed and also install Band lab. All of your SLAT plugs should show up in band lab if you do it that way.

You might want to check all updates of CbB at "Cakewalk by BandLab Release Notes".

Forget about the content bundled with Platinum for a moment, consider just the DAW. CbB is definitely an upgrade. There have been quite a few bug fixes and enhancements this year. The release notes cover the activity.
 
As long as Platinum is installed upgrading to CbB loses nothing compared to Platinum 2017-10. All the plug-ins and other content bundled with 64bit Platinum work fine in CbB.
 
CbB installs as an upgrade meaning it places the DAW, content and config files in their own folders separate from Platinum. The shared folders are overwritten with upgrades. IOW, a CbB upgrade works like upgrading SONAR 8.5 or X3 to Platinum.
 
And CbB is supported. The few times I have contacted support the response has been pretty good.

So, I need to leave the old SPlat installed forever or else the plugs included previously will no longer be in the newer version... or just save my old Cakewalk VST folders, etc..  
Actually I'm seeing the new CbB is sharing the same folders?

It is hard to know the future. For now CbB includes the following plug-ins. BandLab purchased all the Cakewalk code including the plug-ins. They have not provided a release schedule for any additional products.
 
The current recommendation is leave SONAR installed when upgrading to CbB. Both DAWs will be available and everything bundled with SONAR will be available in CbB.
 
The parts that are shared between SONAR and CbB are the same parts that all Cakewalk DAWs share. Each Cakewalk DAW had a folder for the DAW itself, Cakewalk Content, the user directory and programData.

thanks Skook...you've always been helpful.  I'm seeing PC stuff...like all the Boz modules, the CA2A...maybe some other stuff I'm not noticing now..PC4K items...???....that I'm having to import via having Sonar installed.  None of it's deal killer for a fine DAW as the main core, however.
Lastly, I've been trying to figure out how to import my old Sonar themes into the new CbB.  If I roll over in a ditch, I'll come back, but I'm sure there are threads already about this, that I just haven't read.
Any rate, I kinda left Sonar for a bit to digest Studio One, but there's a lot of unfinished music I'd like to revisit, not to mention I've been missing some Sonar items  (Can you believe Studio One doesn't have "sound on sound" recording yet?...except with MIDI) 
Oh...my faderport 8 actually works pretty decent with CbB.  Certainly not as integrated as with S1, but I don't always get too deep into it and just use the faders and transport.

The theme layout is the same. The editor is new and on the Add-On menu

With Platinum and Melodyne installed skip all the other add-ons.
 
The theme file layout is the same but CbB themes are stored in Cakewalk Content\Cakewalk Themes. To see the old SONAR themes, copy the SONAR Themes folder content into the Cakewalk Themes folder. There have been a few updates that SONAR themes will be missing so the old themes will have a few Mercury images and colors.

That all worked. Perfect and thanks!