The most convincing story I have ever heard regarding New Coke was related to me by a professor at the University of Utah during a course in International Political Economy. Aside from his occasional grammar and spelling errors, I had a fair amount of respect for him and his Stanford education. So I give the story some amount of credit. It goes something like this:
The 80's saw a period of transition in the world sweet stuff market. For one reason or another, sugar producers began demanding higher prices from their customers like the Coca-Cola Company. Coca-Cola wasn't too happy about this turn of events, sugar being fairly high on the ingredient list of most of their beverages.
About the same time, corn producers were holding an ace card: corn syrup. It was dirt cheap, and in plentiful supply. They approached Coca-Cola, who saw their chance to give sugar producers the one finger salute and eagerly accepted the proposal.
Believing their customers to be pretty sharp, Coca-Cola felt that they could not just swap the two ingredients without anyone noticing. A sinister plan was devised...they would "retire" Coca-Cola and "replace" it with New Coke. Following the inevitable widespread revolt, Coca-Cola would be resurrected with one little difference, and no one would be the wiser.
And that's what went down, folks. If you don't believe me, try tracking down an old Coca-Cola can and compare the ingredients with the one you bought yesterday. Ta-da! The one simple difference you will find is that the sugar seems to have met a grim fate. Boy did that hurt the sugar market. The upside is...corn mills everywhere no longer have to find old sheds and underwear drawers to stick their corn byproducts. Let this be a lesson to you in free market economics.