The first process for creating High Fructose Corn Syrup (HFCS) was developed in 1957 but the mass industrial production process for HFCS wasn't fully refined (by government funded research in Japan) until 1970. With increased subsidies of corn by the U.S. Federal government in the 1970s, HFCS began being introduced into a wide range if products, including soft drinks, starting around 1975. By 1985 HFCS had replaced sugar in pretty much every soft drink, including Mountain Dew. So we know it was sometime between 1975 and 1985. My guess would be sometime between 1980 and 1984 if I were to narrow it down because I personally remember, as a high school student during that time, all soft drinks getting sweeter tasting during that time and I thought it was weird. I was an avid Mt. Dew drinker back and seem to recall the taste changing when I was in high school.