Sierra Mist is changing its look yet again.

The PepsiCo-owned brand, which has undergone multiple makeovers since launching in 2000, is changing its formula and design, with new packaging that recently began hitting stores. The lemon-lime soda will still be made with sugar instead of high-fructose corn syrup -- a change implemented in 2010 -- but the brand is adding stevia, which will result in a 20% calorie reduction for an 8-ounce serving, a PepsiCo spokeswoman said.

The formula change marks the first time PepsiCo has put a stevia-sweetened soda in the U.S., and follows the recent launch in the states of Coca-Cola Life, which is sweetened with cane sugar and stevia.

Sierra Mist's packaging update includes font changes -- the 'M" in Mist now looks like two triangles colliding. Also, gone are images of sliced lemon and limes riding liquid waves. In their place, the brand is using something a little more psychedelic that meshes circular images of various shades of green and yellow and white.

"The new package utilizes strong color cues to communicate the lemon/lime flavor to bring a more modern approach to the packaging as compared to the traditional lemon/lime wedges," a PepsiCo spokeswoman said in an email. The changes will be accompanied by a new campaign called "Make Interesting Happen" that seeks to "spark the curiosity of the millennial consumer by delivering experiences from the outer edges of mainstream culture," she added. The effort will include experiential, as well as print, digital and radio ads and consumer promotions and retail merchandising.

The agency on the campaign is Van's General Store, New York, which was co-founded in 2012 by actor Liev Schreiber and describes itself as a "hybrid creative collective." Van's General Store has been working in collaboration with Sylvain Labs, a strategic planning consultancy that has been working on Sierra Mist's relaunch, Scott Carlson, a Van's General Store co-founder, said in an email.

In recent years the brand had been using DDB, Chicago, which took over Sierra Mist in 2011 from Goodby, Silverstein & Partners.

The forthcoming campaign differs from recent comparative campaigns the brand had been running, including a TV spot that ran in 2012 that sought to differentiate the brand from "the other guys" with artificial preservatives.

That messaging failed to lift Sierra Mist from a sales funk. Sales volume in 2013 "dropped below 100 million cases for the first time since 2003, far off its 2006 peak of 145.4 million cases," Beverage Digest recently reported.

The redesign comes four years after Sierra Mist made significant packaging updates as it became "Sierra Mist Natural," replacing high-fructose corn syrup with sugar. The brand was the first mainstream soda to make such a change.

By 2011, those graphics were updated to include more green than white so as to not be confused with a diet soda. Then last year the brand went back to the two-word Sierra Mist moniker, removing the word "Natural" in all caps that had appeared just under the brand name.

The PepsiCo spokeswoman said on Wednesday that "Sierra Mist elected to move away from using the 'natural' label on its packaging due to the lack of detailed regulatory guidance on the use of the term."

The Food and Drug Administration does not have a standard definition for what constitutes natural, saying on its website that "it is difficult to define a food product that is 'natural' because the food has probably been processed and is no longer the product of the earth." Still, the agency says it has "not objected to the use of the term if the food does not contain added color, artificial flavors, or synthetic substances."

The ingredients listed in the previous formula of Sierra Mist are: carbonated water, sugar, citric acid, natural flavor and potassium citrate. The new formula lists those same ingredients along with potassium sorbate, which "preserves freshness," as well as purified stevia leaf extract. A 12-ounce can now carries 120 calories, compared with 140 calories in the old version.

The change comes as PepsiCo and other soda makers seek to reach a goal announced Tuesday of reducing beverage calories consumed per person nationally by 20% by 2025.

Stevia might help reduce calories, but its use in drinks in the U.S. has been met with mixed reviews. For instance, Coca-Cola's Vitaminwater recently reversed course on adding stevia to the drink after consumers complained.

Sierra Mist recently teased its change on its Facebook page and was met with a smattering of consumer complaints, including one person who stated that "the old recipe was a nice crisp taste, [and the] new one has a diet aftertaste."

Sierra Mist, whose volume fell 11.6% in 2013, ranks 12th among all soda brands, according to Beverage Digest. Coca-Cola-owned Sprite, the top-selling lemon-lime soda and 6th-largest soda brand, eked out a 1% volume gain last year, according to the trade publication. Dr Pepper Snapple Group's 7Up, the 15th-ranked soda, had a 4.7% volume drop.

Sierra Mist's tumble has come as PepsiCo has spent less money supporting the brand. Measured media spending fell to $4.2 million last year from $18.9 million in 2012, according to Kantar Media.