To at least answer your question, it's both ways. People on this forum who had cars on the GC have had Tesla call them and reassign a car. This probably happens when their exact spec or very similar is available.
For the rest of us, we have to inquire ourselves. I know that there is a RWD Silver/White/19" and a Red/Black/20" both available in Sydney as that's what was offered to me.
To dash any hopes, I just got an SMS saying they had a look through the incoming inventory and no cars with my spec or similar are available.
I am FOREVER stuck on this f**** Stinkship.