Complex sentence: It was moved to Six Flags AstroWorld, where it opened in 1991, and its lift hill was modified from a direct 90 degree vertical ascent to an ascent at a 45 degree angle so that it would be easier to evacuate the lift if necessary.
Split into simpler sentences: It was moved to Six Flags AstroWorld, where it opened in 1991. At that point, its lift hill was modified from a direct 90 degree vertical ascent to an ascent at a 45 degree angle so it was easier to evacuate the lift if necessary.