This has probably been asked many times but I didn't find any specific thread that addresses this question.
 
I just bought the upgrade to the newest pro from X3e pro. Should I install the new over the top of the old, disable/uninstall the old and install the new, or just put the new as a seperate new install? I have many instruments/sound packs that were bought, do I need to re-install them in the new version or will a version upgrade overlay do that automatically?  Or, will the new upgrade control panel I have read about automatically do what is necessary?
 
I just want to make sure before I commit to a path that may be wrong...

If you install it with its defaults such as it location you wont overwrite anything except some shared components. There is no harm in doing that. You don't need to uninstall anything. Plus if you have some useful plugins from X3 they will stay and be available to Sonar 2015. 

If you do plan to use X3e every again or C: space is a premium uninstall it then install SONAR Pro.
 
If you want to transition leave.
 
Recognize that uninstalling a previous version once a later version is installed can be problematic as the two versions definitely share some common components.
 
 
 

Thanks for the quick responses! An overlay it is!

On my next "fresh" install, I plan to only install SONAR Platinum (all the X seris will go away as I have not needed to open them in the last 11 months).

DrLumen
Thanks for the quick responses! An overlay it is!

No. It's not an overlay as you call it. Each Sonar version installs to its own folder in the Cakewalk directory. ALL versions of Sonar can coexist quite happily.

bapu
On my next "fresh" install, I plan to only install SONAR Platinum (all the X seris will go away as I have not needed to open them in the last 11 months).

I'm down to X2 (for VVocal) and Platinum.

I did an install of 2015 pro in a new folder and most of it works. The SoundCenter synth is causing some serious issues. Almost like it is stuck in a feedback loop. SC works fine in X3e but not 2015 Pro. Rapture Session won't recognize the sustain pedal.
 
I'm probably going to do a complete uninstall of everything and re-install Pro. While this may turn out to be a small issue, it is a discouraging start. (sigh)

The SoundCenter problem is a bug in Kingston.  You need to turn off 64-bit processing in the preferences to work around it.  There are other threads discussing this.