(It also seems to contradict your distinction between "addition" and "alternation" - [MENTION=22779]Hussar[/MENTION] and I have pointed to so-called "additions" that alter the feel of things for us - in my case the incorporation of settings into Spelljammer and Planescape - and have been told that we have to suck these up as non-changing additions.)
I must admit that I am curious about what you feel Spelljammer changed?
Was there a Greyhawk supplement that mentioned what space was like at all?