I upgraded on my own dime, so I'm wondering how I get the additional bits without registering a second version of AD2.

I also previously upgraded to AD2 - all I had to do was register the AD 2 Producer Bundle serial number I got from Cakewalk on the XLN website then choose my items. Once I did that I ran the XLN Online Installer and it updated everything.

If you already have AD2 essentially all that happens is you get the new packs which is a real boost!

 I went into the xln downloader, put in the serial numbers for the addition there...but did not install XLN AD2 through command center...(As I already have this product, and bought the full Melodyn product also, so I did not install that.)
I got three pack picks from a selectable xln window...And then hit the install ...the window for my account opened , and then you see what is needing updates and what can be installed...Just make sure it is all correct.
 
Hope this helps...and updates for Platinum have been released...Thanks to the support of the brain child's R&D folks on the Forums...and we pay for that now....wink.

I  found the whole thing to be more difficult than ever, but my 3 new AD2 packs showed up (yes they are sweet) and have my kit pieces, but after selecting my midi-packs they're still showing up as demos and that's my big current frustration.
 
I'm going to email XLN and get that straightened out, but I'm shutting myself down for tonight.
 
Haven't spent enough time with Sonar-new-thingy to comment yet.