I did exactly this too. It was $50 for SQ80 + $99 for the V6 to V8 upgrade.
If history is a guide, V8 + SQ80 owners will be offered a higher V9 upgrade discount for having also purchased SQ80, which will be one of the new additions added to V9.
I believe V7 + OBXa owners got an additional $50 off the V8 upgrade at intro.