As far as I`m aware, as part of Kodaks contract renewal in 1997, they stipulated the ride be changed and updated. We still havn`t figured out why upstairs was really closed (though it may not remain like that for much longer) but do know some of the reasons the updating removed some of the less reliable parts of the original ride system;
the rotating flight to imagination scene was going to need major repair work. Contrary to popular belief, there was no turntable, more a framework of show sets, rear projection screens and props some 80ft across that rotated on a central base and ran on railings. This was rumoured to be warping under it`s own weight.
The ride system was too clever for it`s day - cars would slowly buch for load, then alter speed an split like a WEDWay for the ride. Car speed and pacing was never fully under control as planned.
Cars also would fail to align properly with the rotating flight scenary. The ride would shut down regularly - normally a few times a day. At best a cascade stop was triggered, at worse the cars could and did derail when empty.
There was also interface issues with the unload area - the original unload moving belt was removed shortly after the ride opened (alte) because of this.
All in all, a very clever ride that was a little too advanced (as in Test Track??!) Imagine what could be done with the same idea using todays technology.