And
@MrConbon makes an interesting observation that Spaceship Earth is changed every so often and usually the change is warmly received while a change to Haunted Mansion or Pirates causes a riot. And perhaps that could be one way (of MANY, I mean.. we could always just call it an Epcot classic when it was in Epcot of course) to distinguish an Epcot classic from a standard Disney classic. Classic Epcot attractions REQUIRED changes every few years to keep their overall purpose in place. However, Disney quickly saw how that became impractical due to the high costs and how much downtime the rides would have and that contributes to why there are very few classic Epcot attractions left. I would guess that Spaceship Earth has probably only been spared the same fate as Horizons, World of Motion and Universe of Energy because of the fact that it's in the icon of the park.
Meanwhile, the standard Disney classics go decades without a single change and it's not due to the attractions not receiving proper maintenance (different can of worms) but it's because they often struggle to find out how they could improve it. For example, every time I go on Haunted Mansion with someone who's never been to Disney before I love asking "are these projections?" and then explaining pepper's ghost effect and how projections would probably look very out of place in the ballroom. Therefore, pepper's ghost works just fine and probably is better suited for the scene than modern day projection technology and if they ever did switch over, it would probably not be a welcomed update.