Here's what happened with Coke:
Coca-Cola was making their original product, and when they introduced a Diet Coke, they formulated it to taste as close to the original, but because it was a different, artificial sweetener it was a completely different recipe.
In the 80s Diet Coke was outselling Coca-Cola, so they figured to save money they'd standardize on one formula, but with different sweeteners. So one batch would get aspartame, and while making the change they also switched over to high fructose corn syrup in the US for the other batch. The later became New Coke.
After much protest they reintroduced Coca-Cola Classic, which went back to the original formula, but stuck with the HFCS instead of cane sugar.
If you liked the taste of the real original Coke, get some from Mexico (or Romania, where I drink tons of Coke when I visit).
