Cakewalk by BandLab exists beside your present Sonar products. The program installation will recognize and allow you to use whatever existing third party products that is part of your current Sonar installation.
The starting point for Cakewalk by BandLab was the core executable program for Sonar Platinum. That means you'll gain the artificial program limitations imposed to create the Artist and Professional program variations. For example, looking at the comparison chart available
+++ HERE +++ Sonar Artist does not include the ProChannel while Sonar Platinum did so Cakewalk by BandLab does.
Cakewalk by BandLab will use whatever third party content you installed with Sonar Artist. However, content for Cakewalk by BandLab is limited at this time to the Studio Instruments suite, TTS-1 and the trial edition of Melodyne Essentials.
There have been two updates for bug fixes, ease installation concerns and preparation for future updates.
The advice is to keep what you have for now. There will come a time when the new program does not depend on the old program accessories so much but keeping the installation you have is the only way to ensure you will not lose any functionality by accident.
One last caution: Cakewalk by BandLab requires 64 bit Windows 7, 8, 8.1 or 10. 32 bit is not supported.