Actually it's a different processor and memory. Here's a link that explains it.IDK about not being able to upgrade, its android based so that really shouldn't be an issue. The only hardware changes are wireless carplay/android auto, more pixel depth and a slightly larger screen. They are still going to write updates for the current 12" so my guess is that they'll write code specific to the 12.1" and the 12.3", the same way they do phones.
Uconnect 5 really doesn't add anything I care about though, I dont use android auto now.
Explanation of Uconnect 5 Hardware