THERE AIN'T nothing like the real thing, baby.

Not any more there ain't.

Coke is changing its formula. After 99 years, the Coca-Cola Corporation has decided to put more sugar in the world's favorite soft drink.

The move is designed to head off Pepsi's increasing success with its viscous, sticky-sweet product. While the market-share figures don't indicate an imminent end for Coke, the trend in the last five years has been toward Pepsi.

In 1980, according to First Boston Inc., Coke maintained 24.3 percent of the soft drink market and Pepsi held 18 percent. At the end of 1984, Pepsi was up to 18.8 and Coke down to 21.7.

On the whole, considering all of its products, the Coca-Cola share of the market has grown from 34.4 percent to 34.6 over the same period.

But regardless of that increase, which is due in large part to the success of Diet Coke, Diet Sprite and Sprite, the powers that be in Coke's Atlanta corporate headquarters have decided to sweeten Coke.

Let's stop them.

They have no right to change Coke. Sure, they may own the pieces of paper that give them legal authority to do what they will with the soft drink.

Those same pieces of paper may even give them the right to revise the Coke formula and control product marketing, but Coke can no longer be considered the specific property of any corporate entity.

Coke, like apple pie and mom, belongs to all of us.

Would we tolerate it if America's mothers gathered together and announced that because we had been eating a little too much pumpkin pie recently, they--as a group--were going to dump an extra half-cup of sugar into every one of their apple pies?

No, moms know that they don't have the to change grandma's recipe--they wouldn't dare.

Major league baseball wouldn't give teams four outs per inning in order to increase scoring, to draw more fans to the park and thus to combat the surge in the popularity of football.

Baseball fans know that football is only a fad; they jealously guard the integrity of the national pastime.

For the same reason, Coke shouldn't mess with its glorious past.

Coke is now inextricably woven into the very fabric of 20th century American history. All of our fathers started taking all our mothers out "for a Coke" at the corner soda counter. GIs thoughout our history have punctuated their between battle rest with an ice-cold bottle of Coca-Coda.

Flea markets are replete with old Coke trays and posters. The 10-ounce Coke bottle is a paragon of American pop art.

Russians drink Pepsi, Americans drink Coke.

FURTHER, the real reason for Pepsi's recent market surge can be spelled M-I-C-H-A-E-L.

Taking a cue from the old, familiar "I'd like to buy the world a Coke" theme. Pepsi bought the world a superstar. And ever since Michael Jackson and Lionel Ritchie have been plugging Pepsi on TV to starry-eyed youngsters, sales have been up.

But like Pepsi, Jackson and Ritchie are fads--flashes in the pan "Thriller" is no American classic, but Coke is.

To compete in the short term. Coke has only to hire an androgynous superstar of its own.

"This is Prince and I would die for you, so believe me when I fell you Coke, the official soft drink of short, sexually ambiguous Minneapolis rock stars, is it."

Well, on the other hand, maybe Coke is right to sit tight with its present spokesman, the incomparable Bill Cosby.

After being off the air for a long time, Cosby came back to prime time last fall and his sit-com vaulted into the top spot in the Nielsens midway through the season.

Nothing sells like quality.

It's a lesson in patience Coke could learn. But rather than sit back and count on what may be the greatest product in manufacturing history. Coca-Cola's pouring sugar into America's soft drink.

Coke is due to turn 100 years old next year, but it's a birthday it'll never be able to celebrate.

It just died.