Are you ready for some football? And aspartame-free Diet Pepsi?

PepsiCo hopes that consumers answer yes to both of those questions, as it uses its National Football League sponsorship to help launch the reformulated soda.

The brand announced plans to remove the much-maligned aspartame sweetener in April and replace it with sucralose combined with acesulfame potassium, known as Ace-K. But cans and bottles won't start hitting stores until Monday. With football season around the corner, much of the marketing plugging the new version will take advantage of the marketer's pricey NFL sponsorship.

"We want to be able to leverage one of our biggest partnerships with one of our biggest pieces of news," said Seth Kaufman, senior VP for the Pepsi and flavors portfolio in North America.

One ad running in stores and on digital channels shows an NFL football and describes Diet Pepsi as "Crisp, refreshing -- now aspartame free." But PepsiCo does not have immediate plans to plug the new version in TV ads. And the initial aspartame-free ads running elsewhere are are pretty basic, free from celebrities or other flashy tactics. To use a football analogy, PepsiCo appears to be focusing on basics like blocking and tackling.

That is a departure from the last time Diet Pepsi changed its formula in 2013, when it blended Ace-K with aspartame, which had been in use since 1983. The 2013 change was accompanied by a major ad campaign called "Love Every Sip" that starred Sofia Vergara.

But since then, Pepsi has shifted to a masterbrand approach whereby Diet Pepsi no longer gets dedicated TV campaigns. "Diet consumers are a very, very specific consumer segment. And digital is a great way to get extremely, extremely targeted," Mr. Kaufman said. "So that's why we want to lean into digital in a significant way."

Aspartame's negative perception has contributed to falling sales trends for most diet brands. Consumers and some activists have blamed the sweetener for contributing to various health issues over the years.

Diet Pepsi sales volume dropped 5.2% last year, while Diet Coke sank 6.6%, according to Beverage Digest. Diet Coke has stood by its use of aspartame, calling it safe. But Diet Coke for the past 10 years has also marketed a version with Splenda that uses a formula combining sucralose and Ace K.

The American Cancer Society's position is that "for most people, no health problems have clearly been linked to aspartame use."

But as consumer concerns rise and spread on the internet, PepsiCo has walked a tightrope. The marketer has defended aspartame as safe, but said the formula change is in response to consumer demand. So it is not surprising that the ads touting the new version avoid the thorny health issues and focus on taste.

"Our Diet Pepsi consumers expect a crisp and refreshing taste," Mr. Kaufman said. "It is important that we emphasize to those consumers that this delivers against that taste."

Asked about the exact taste difference between the old and new version, he said the "core profile … is really, really similar. If you are asking does it taste exactly the same where no one can tell? No. But a lot of consumers think it tastes better."

He added that the wild cherry version of Diet Pepsi -- which is now also aspartame-free -- "by a landslide tastes better." The marketer's confidence in the cherry variety, as well as its general bullishness on flavored cola, is why the cherry version will be featured prominently on in-store signage for the first time, he said.