Having been a long-life Great America attendee and afficianado, and having spoken to Park Operations people at both parks, the Turn of the Century was turned into the Demon as an attempt to make the coaster "more exciting." Keep in mind, that the year this was done, 1980, was around the time the loop was becoming "en vogue." Mariott's wanted to add something 'new' that year, and instead of forking out money for a new coaster, they put it into the loops, flashing tunnel (with red, flashing lights, and blood-tainted water that splashed up on you), and the faux caverns, etc. The 'new' coaster opened to a mega fireworks display in May of 1980, along with some excellent television commercials. Do you remember them? "The newest ride at Mariott's Great America...The nine story drop...Double loops....Tunnel....Two corkscrews....New at Mariott's Great America."
Apparently, what they did was a hit. Ridership increased significantly on the Demon, and it became the park's signature coaster for awhile. In fact it was the ONLY multi-looper at the park until Shockwave was introduced in 1988.
Funny, looking back, with its airtime hills, and smooth ride, Turn of the Century was really ahead of its time, at least by today's standards. But the pleasure of going upside down, into dark, disco-like tunnels with fun sound effects, made the ride more exciting and perhaps more palatable to the younger crowd and general public.
Much of the original TOTC track was retained, just repainted. Except, of course the sections for the loops, etc.