Some part of me thinks that this was all the work of a very gifted maintenance tech guy who has a deep passion for Epcot. Over the last few month's he's been sneaking away on his breaks to maintain these animatronics and get them looking good and into shape to be installed. Then, when the time is right, he goes in, removes the screen and plops in the animatronics and programs everything to work properly without anyone seeing. Next day the guests notice something different about the ride and the CMs are all confused. The suits have no clue it's been added because they don't leave their comfy chairs and the maintenance guy tries hard to hide his gleaming smile as he hears word of how everyone loves the plus. He starts to plan his next ride that he will secretly plus in the cover of darkness as people come up with a name for his secret identity.... "The Night Doctor" in reference of how he is bringing Epcot back from the dead, one surgery at a time.
It makes for a good story at least.