Coca-Cola, which brought back Coca-Cola Classic after a consumer backlash, is now facing another challenge from some hard-core cola-heads who insist even the revived old Coke isn't "The Real Thing."

At issue is the use of high-fructose corn syrup in place of sucrose (beet or cane sugar), a practice that has become common throughout the soft drink industry over the past five years or so, because HFCS (as it's called) is sometimes 5 to 8 cents a pound cheaper. The National Soft Drink Association said yesterday that most companies are changing their labels to show the change. The new Coca-Cola and Coca-Cola Classic labels list high-fructose corn syrup and/or sucrose.

If you're willing to wait until next spring, you can drink a beverage as close to old Coke as you're likely to get. The Mid-Atlantic Coca-Cola Bottling Co. each year returns to all-sucrose Coke for Passover. Corn syrup, like leavened bread, just isn't kosher for Passover.

Sue Challis, Giant Food Inc. spokeswoman, said the company has had no complaints about Coke Classic since Giant stocked it in its stores last Wednesday. Other stores have reported booming sales of Classic. A spot check of Peoples drug store managers, most of whom said they'd tried it themselves, uncovered no unhappy Classic customers.

But there are Coke drinkers who aren't content.

Lois Mintz of Annandale said, "I'm an old Coke drinker for 35 years. I still have some bottles of the old Coke at my house. The other afternoon, I tasted the Classic Coke and I said, 'It isn't the same.' The Classic Coke is flatter, it doesn't have the sharpness of the old Coke."

Rhonda Golder of Silver Spring said, "After I put Classic Coke in my car, I read the ingredients and the second one was high-fructose corn sweetener. The old Coke said sugar. How can it be the same if the sweetener is different?"

Gay Mullins of Seattle, founder of the Old Cola Drinkers of America, said the taste of Classic Coke "was good, but the aftereffects . . . The high fructose feels like lead in my stomach. I get sick after the second can. I couldn't honestly promote it. It isn't the same thing."

So far, Mullins said, he's received "50 to 100 complaints about Classic Coke including one man who had to go to bed after drinking three cans."

Sarah Setten at the Sugar Association Inc. said the group, which has been complaining for several years about soft drinks switching to high-fructose corn syrup, is mounting a new campaign. Today and tomorrow, the group plans to run a full-page advertisement in 12 newspapers across the country with the headline "Why Did the 'Old Cola Drinkers of America' Turn Up Their Noses at Classic Coke?"

The advertisement goes on to say that "They were right. For 94 years Coca-Cola was in fact 'The Real Thing' -- a classic sweetened with real sugar -- an unvarying taste standard known and trusted the world over. But five years ago, Coca-Cola quietly began to change its formula. Until 1979, the Coca-Cola Co. used only sugar as a sweetener. In 1980, the company began blending corn syrup with sugar, using as much as 50 percent of the substitute. In 1984, it went to 100 percent use of the substitute and no sugar. At no time during this five-year period did the company advertise these changes to the consumer or indicate them on the Coke label."

Tony Tortorici of Atlanta, director of public affairs for Coca-Cola USA, said by phone that Coca-Cola Classic is "exactly the same as the original formula for Coca-Cola." However, he confirmed the Sugar Association's history of the percentage change from sugar to HFCS, adding, "It has absolutely no effect on quality or taste."

Julie McCahill, vice president of the National Soft Drink Association, said it had "no complaints or questions about the high-fructose corn syrup." She added that Seven-Up is now using a label showing a combination of sugar and corn syrup. "Pepsi has its label change in the works. RC will soon. Other companies will follow suit."

She said that Howard Roberts, the association's science and technology vice president, was not aware of any allergic reactions to the high-fructose corn syrup. Insofar as McCahill knows, no major soft drink company still uses all sugar.

Setten said the Sugar Association wrote the Food and Drug Administration two years ago and later petitioned the agency to require soft drink manufacturers to change their labels to indicate the use of HFCS. In August 1983, Joseph P. Hile, associate commissioner for regulatory affairs, sent a letter to the National Soft Drink Association saying: "The ingredient declaration of 'sugar' on products containing non-sucrose sweeteners not only is in violation of the act and the regulations but also fails to fully inform purchasers who may be allergic to certain other sweeteners or consumers who may simply wish to avoid certain ingredients for whatever reason."