You guys are going to kill me for not knowing this, but here goes.
 
• I am on X1d Producer (64-bit)
• I want to upgrade to the most recent and most complete version of Sonar, which I assume is Platinum (and which I think Brits call Platuminium).
• Question 1: do I just go to the Cakewalk store, give them money, and get some kind of install package that will be self-explanatory without having to install interim/previous versions?
• Question 2: Will all my VSTs/instruments magically work?  I have had a lot of trouble in the past getting dlls in the right folders and scanned correctly, particularly if the VST in question offered both 32 and 64 bit versions.  If those file locations remain untouched, that would be good.
 
If Ivory II, RealGuitar, Kontakt, and Vienna don't work, I will have to jump off a tall building, after which you will all be subjected to countless tributes to my musical genius, fans lighting candles, TV retrospectives, and all that.

It's supposed to work like that. All your plugins from previous installations should still be there and working. If you have in the past put things in custom (non-factory) directories (and you like it that way), don't use the Cakewalk Control Center. Just download your new software from your store account and install manually. Best of luck.

Thank you, sir.

You're confusing Platuminium with Aluminium

Bristol_Jonesey
You're confusing Platuminium with Aluminium

That`s the polite way of explaining it 

  Sorry friends across the pond.  I was just being silly.

Eye no...

Good luck Konradh.

I would do as Larry suggest and d/l the install pack since you have custom folders.

I had custom folders on my home PC but let CCC install away. I thought it would ask me where to go with stuff. It didn't. Now i have to uninstall everything to put it where I want. Not my music PC so not really worried about it. But, still. Didn't know this beforehand. : (

However, on my midi PC I used the install packs and everything is just as I want it. Had no problems that I am aware of.

Glad I asked!  Thanks, guys.

http://forum.cakewalk.com/Using-Command-Center-a-Guide-m3183900.aspx 
 
This will walk you through the installs.