There's very few, if anything, original to it's debute. Even Snow White got changed. The Country Bears is about the only thing that hasn't seen a change, but they did add the christmas show and summer vacation hoe-down, which both are gone, so it's back to the original.
The Oddysse building at Epcot is still in use. They use the kitchens to make the pizzas sold in the cart outside of test track. And the Grand Gathering dinners are held there. As well they usually house special events from the Food and Wine festival and Flower and Garden festival.
Even ToonTown used to have the stage show in the big tent before it got converted over to Character Meet and Greets. You can tell because of the way the floor dips and curves, only certain areas were filled in as needed for leveling.
The Tomorrowland end of the Sky Way ride has been recently torn down. Aparently they used to house the gandalas in it, but who knows what they did with it.
They took out the Galaxy Palace Theater to make more parking spaces and expand the Outdoor Foods storage building. Put in that stupid little stage for the Stitch show which got canceled after like a month... no pre thoughts into doing that one...