Well, according to Snopes this is a myth; bottlers had been allowed to use HFCS in original Coke prior to the introduction of New Coke. Here's what Wikipedia has to say:
The new product continued to be sold and retained the name Coca-Cola (until 1992, when it was officially renamed Coca-Cola II), so the old product was named Coca-Cola Classic, also called Coke Classic, later just Coke and for a short period of time it was referred to by the public as Old Coke. Many who tasted the reintroduced formula were not convinced that the first batches really were the same formula that had supposedly been retired that spring. This is partially true because Coca-Cola Classic differed from the original formula, as all bottlers who hadn't already done so were using high fructose corn syrup instead of cane sugar to sweeten the drink
The new product continued to be sold and retained the name Coca-Cola (until 1992, when it was officially renamed Coca-Cola II), so the old product was named Coca-Cola Classic, also called Coke Classic, later just Coke and for a short period of time it was referred to by the public as Old Coke. Many who tasted the reintroduced formula were not convinced that the first batches really were the same formula that had supposedly been retired that spring. This is partially true because Coca-Cola Classic differed from the original formula, as all bottlers who hadn't already done so were using high fructose corn syrup instead of cane sugar to sweeten the drink