> I think what you end up with is a one-kit version of AD2 and they don't give you new versions of the Pop/Rock/Prog kits. I'm not sure about that, this is all pure speculation from people who haven't bought the product in this configuration, or can't supply the full specs (next to useless then and probably adds to the confusion).
I think you
might just get away with it (but again this is useless advice).
I would have contacted the Walk of Cake first myself so good luck :).
The confusing "engine" terminology must be the app (damn it marketers please note it's an application with an engine within it

) as the upgrade is also referred to as "engine" here:
http://www.xlnaudio.com/upgrade/ Cheers ! :)
(
Edited to insert a missing quote)