The conspiracy theory, which is far-fetched at best, goes something like this:

Coca-Cola wanted to slightly change the time-honored formula of their original product. We'll call the original Coke A and we'll call what they wanted to change it to Coke B.  They know that if they make the change directly, people would notice.  So, they concoct some awful-tasting soda (New Coke, or Coke C in this story) and replace Coke A with it.  They stay with Coke C until all traces of Coke A are out of the marketplace and out of people's homes.  Now, "bowing down" to popular demand, they then replace Coke C with Coke B, NOT Coke A.  An interesting note is that this version was (and still is) marketed as "Coca-Cola Classic," not simply as "Coke."  Coca-Cola will tell you this is because they intended to still sell New Coke and didn't want consumers to get confused, but we know that it's really because it was a different product than the original Coke A.  No one's had the original Coke A for a year or so, and so the memory of what Coke A was has faded just enough for no one to realize that Coke B is not actually Coke A.  Thus, the switch is complete, and the original Coke A hasn't been sold since 1985.

It's probably complete tripe, but it's funny.