Out with the "O" and in with the heart.

General Mills is changing the most recognizable shape in the cereal kingdom, the "O", to include heart-shaped pieces in its two most popular varieties of Cheerios. It's all a part of the food maker's ongoing marketing campaign that claims its whole-grain cereals can be good for heart health.

But fret not, fans. The change is temporary. Boxes of the limited-edition version — which will have a mix of hearts and the iconic Os — are starting to appear on store shelves and will be available nationwide in January ahead of National Heart Health Month, which is February.

The hearts will appear in Honey Nut Cheerios and the original yellow-box Cheerios — the Nos. 1 and 5 breakfast cereals in the U.S.

It's the first time the company has changed the shape of its Honey Nut Cheerios. It has twice changed the original Cheerios shapes for limited-edition releases since it was launched under the name "Cheerioats" in 1941. The brand is now the most popular ready-to-eat breakfast cereal in the nation with dozens of variations appearing on store shelves through seasonal, temporary and permanent releases. General Mills has the largest market share in U.S. cereal at 32%, according to Nielsen.

Interestingly, Kashi — which is owned by Kellogg Co. — makes a cereal similar to Honey Nut Cheerios, called Honey Toasted Oat, which also has both "O" and heart shapes. Unlike Kashi's hearts, General Mills' new hearts have a hole in the middle, like a doughnut.

In announcing the shape swap, Golden Valley-based General Mills noted more than 100 million Americans live with some form of heart disease. General Mills knows who is eating which of its cereal brands and why. This heart-healthy message is one of the company's big cereal campaigns for the year.

"It's all about the boomers with new heart-healthy messaging on Cheerios," Jon Nudi, General Mills' president of North America retail, said in September at the Barclays Global Consumer Staples Conference.

According to the news release, this shape-shifting promotion will be joined with other messages encouraging consumers "to adopt simple, fun behaviors that get them active like walking the dog, playing with grandkids, or joining a group fitness class."

Oh, "and of course, eating 3 grams of soluble fiber daily from whole grain oat foods, like Honey Nut Cheerios, which can help lower cholesterol as part of a heart-healthy diet," General Mills said.

General Mills recently won a legal case in California over these heart-healthy claims. A San Diego-based law firm known for bringing consumer-protection suits against companies filed a class-action lawsuit against General Mills, alleging its heart-healthy claims were false and misleading.

The complaint argued that Honey Nut Cheerios' sugar levels disqualified it from such claims.

The plaintiffs pointed to links between a sugar-rich diet and negative health outcomes, such as heart disease and Type 2 diabetes.

But the judge dismissed the case in August, noting that General Mills provided consumers "with all truthful and required objective facts about its products, on both the side panel of ingredients and the front of the products' labeling. … It was up to the plaintiffs, as reasonable consumers, to come to their own conclusions about whether or not the sugar content was healthy for them."

The issue remains active within the packaged food industry with the company's lead rivals, Kellogg and Post, both having faced similar lawsuits in recent years.

"Honey Nut Cheerios is on a mission to inspire happy hearts," Liz Mascolo, vice president of marketing for Cheerios, said in the release. "We hope the new heart shapes not only put a smile on people's faces but inspire them to find new, fun ways to take care of their hearts."