Is this (or was it) ever possible?
Well … I thought that I’d have a clear answer for you after visiting the Steinberg shop - http://www.steinberg.net/en/shop.html - but was surprised to see that there were no options for a crossgrade. So maybe a moderator needs to chime in on this. However to answer half of your question, as I recall it was indeed possible a few years ago to crossgrade between Nuendo and Cubase for a fee.
I’ve often wondered about that but can’t recall having ever being able to cross grade?
At least a drum map would be nice.
In fact I’d love it is SB would create additional maps, such as guitar tab using XML and sell them as new features.