When you install the new Cakewalk, you have the option of installing just the application or app + content. If you already have SONAR Platinum installed, there is no need to install the content.
My C: drive is only 256 GB in size, so the bulk of my audio resides elsewhere.
I have all my CW instruments (AD, Studio Instruments, Dimension Pro, Rapture, Session Drummer, etc.) on my E: drive, which is also where all my other samples live (e.g. Kontakt, Superior Drummer, Omnisphere). I had previously set up junctions on the C: drive so that, from SONAR's perspective, these appeared to still be in their default locations. That saved having to reconfigure individual instruments. Those links are still working fine under the new program.
After installing New Cakewalk, all I had to do was go into preferences and correct a couple things: added some folders to the VST scan paths, and changed my Projects and Wave Files destinations (on my system, they are d:\projects and d:\exports respectively). After those minor adjustments, I was able to open and edit existing projects in either application and even bounce back and forth between them. You can even have both open at the same time, which is handy for comparing settings.
The new cakewalk.exe is, at this time, merely a recompile of sonarplt.exe with some enhancements. For now you can think of Cakewalk by BandLab as the next version of SONAR.