Request for Installation Guidance: Upgrading X3 Pro to Platinum

If you can afford it, a larger SSD would defer the problem for some time, possibly forever.
 
Letting the Command Center handle updates is the least painful route. Using the verbose setting and alternate paths provided in the Command Center configuration can also help, if you choose to install things in other than default paths. Relocate the download path and/or removed the installers from the path after installation.
 
Just as with every version of SONAR there is a tremendous amount of overlap between X3 and the new SONAR. More than half the disk space is used by samples and audio loops. There are no changes in the sample libraries which install with the sfz synths (DPro, Cakewalk Sound Center, Rapture Session Drummer and Studio Instruments). I would not mess with uninstalling the previous version, it is not going to return much disk space because of the amount of shared data between releases.

chamlin
  A.  Which of these should I NOT change from S: to C: (other than Project Files)?

Here is how I setup a second internal drive as a project drive. With SONAR not running

• Rename "C:\Cakewalk Projects" to "C:\Cakewalk Projects 2"
• Then run cmd.exe as administrator and type
  • mkdir "S:\SONAR Projects\Audio Data"
  • mkdir "S:\SONAR Projects\Picture Cache"
  • mklink /j "C:\Cakewalk Projects" "S:\SONAR Projects"
• Copy any files in "C:\Cakewalk Projects 2\Audio Data" to "S:\SONAR Projects\Audio Data"

If you work with video that path probably should not point to the system drive. Everything else can work with the defaults on the C: drive although I prefer to some custom locations. For example, I use a custom location for drum maps and project templates in order to keep those lists small. Once you are sure everything is running OK, delete or archive the old "Cakewalk Projects 2."
 
If there is a third drive available, I would put the picture cache on it otherwise, leave it alone. It is handled by the instructions above.
 

chamlin
B.  If I change these will Sonar automatically adjust this for all existing projects, or will projects be  confused as to where things are?

Projects which use the default layout of per-project audio folders, IOW with an audio folder nested below the project files, are portable. The projects use a relative path to the audio data. This means you can move the projects anywhere and as long as the data stays in the same relative location, nested in a folder below the project files. If you are currently using the global audio file, copying the data to the new location I mention above "S:\SONAR Projects\Audio Data" will work for them. Of course the data will need to be moved before removing the "C:\Cakewalk Projects" folder. If you use another scheme you will probably have to tell the project when the audio files are located when you open the project.
 

chamlin
C.  Do I need to move the actual content of these folders or will Sonar automatically do so?

When changing paths in preferences, you are responsible for making sure the data is in the correct location.
 

chamlin
D.  How can I safely move that audio data to the proper folder on my S: drive?

Covered in the answer to B above unless this is a question about how to convert project using global data to per-project audio folders. The conversion is performed by opening the projects and using the "Save As" command with the "Copy all audio with project" enabled and the paths properly defined in the dialog.