Surely this has been asked before, but I'm not finding it through search.
 
I'm considering a total rebuild of my computer. Do I have to reinstall all versions of Sonar in order (starting with X1) before I can install Platinum, or can I just prove possession of the previous versions, and install Sonar as new?

Yes (to the subject line question). All upgrades with the exception of the SONAR 8.5 download upgrade are full retail versions. Some plug-ins and content may be discontinued from version to version but each upgrade is a complete version. Upgrade in this case refers to special pricing only.

Thank you, scook. That makes life so much easier. X1 can just sit in it's box. :)

You might want to take a look at http://forum.cakewalk.com/FindPost/3217211 for the plug-ins not included in Platinum. To add missing plug-ins use the advanced install option of the previous versions. V-Vocal is a special case which requires installing the actual SONAR program to get the plug-in. The rest are separately selectable in the custom pick list during an advanced install.