OK, You are right, there is Nuendo 8 Crossgrade from Cubase. And it’s also clearly written here:
Please note: This crossgrade replaces your Cubase license with a Nuendo license. Note that this license will no longer run Cubase.
he same is not written in the Nuendo 8 Crossgrade from Nuendo Live.
All other crossgrades are from 3rd party, as far as I can see.