They did rip out the entire track and replaced it with a smoother tubular steel track. This allowed them to redo the onboard stereo systems that kept crapping out. The old track could also not support the additional weight of the current car remodifications - that's why it became as rough as it was. The design of the track was not changed, just the track materials.
Also, they resurfaced the entire surface of the interior dome - sealing any light leaks caused by age and building settling. Then, they repainted and enhanced the special lighting effects.
That's why it took 18 months to do the refurb.
And the folks going to WDW are whining about an 8 month closure, when their SM needs rehab even more....