Walt was for change, but I think he was also about keeping things that worked and just plussing them when he could. Look at the Jungle Cruise or the Mine Train Thru Nature's Wonderland. He just kept adding to them to make them better, year after year. For the most part, he really only bulldozed attractions when they were something that wasn't working (Disneyland Circus) or something that was done as a stopgap measure to begin with (Hall of Chemistry and much of the rest of the original Tomorrowland).
At EPCOT, we've lost just about everything that was originally designed for Future World. I think the only two attractions that haven't been seriously altered are Spaceship Earth (an instance of plussing in the Park, in my opinion) and the Living with the Land attraction. In one case I think the major overhaul was actually a good thing - Universe of Energy. I rarely use emoticons, but this one says it all in regards to the original UoE - :snore: And, unlike most of the new attractions, it includes an element of the future in it. It asks where are we going, what can we do to improve the future, etc. These are questions almost all of the original Future World attractions asked at some point - Horizons, World of Motion, The Living Seas, Spaceship Earth, and yes, even the original UoE. The new attractions seem to be more of an appeal to the so-called MTV generation - short, quick rides, instant gratification, and a lack of plot. Call me an old fogey, but I honestly prefer attractions like Horizons, World of Motion, and the original Journey Into Imagination that take 15 to 20 minutes, educate me, entertain me, and actually leave me feeling awed and inspired. Maybe that's what it all really came down to for me when I first visited Future World - Awe and Inspiration. I hope EPCOT gets to that point again someday (if we can dream it, then they can do it), but for now it just doesn't hold the appeal for me it once did...not that I'll go there any less (Spaceship Earth and American Adventure still exist!), it just doesn't have the one little spark it used to.