WDW Space Mountain was to get a full track replacement and additional supports. That and the new fleet of rockets were cut so late in the design process that the cut marks on the ride were spraypainted! What we got instead was a few hundred feet of track replaced, sanded track and a darker ride throughout. The track now has degraded to the point they had to install magnetic brakes to slow the rockets down before the big drop. The turn and helix at the bottom of the ride needed to be protected and obviously can't take a full speed rocket anymore.
WDW BTM was to get a full rebuilt as well... but changed to sanding out the rough spots of the track and refurbing the old trains.
DL BTM and Space Mountain were rebuilt by Dynamic Attractions. BTM was fully replaced, but the lifts on Space Mountain were preserved and used on the new 2005 Space Mountain.
DLP Space Mountain has had a few track sections replaced. The next step is a new fleet of Vekoma MK-1212 shells for the trains with vest restraints. Vekoma brought a MK-1212 shell there a few years ago, installed it on a train and they ran it for a few lucky people afterhours. The result was positive and a great improvement, so those trains are coming. This is the MK-1212 standard shell and WDI showed earlier this year a few concept arts of what they will do to them for Space Mountain.