If you're a Diet Coke fan, expect to see new packaging, advertising, and general marketing. And if you're a millennial, better get set, because you're in the target group of the new campaign rollout that starts today.

Sometimes you wind up with a brand problem, like when you let politics inadvertently pulverize your customer relations. A fix requires more than great design. It may need some ingenious subtlety.

And you have to avoid losing your customers in the process, even when you want to expand beyond them.

As AdAge noted, Diet Coke sales were down 3.7 percent last year. Originally, as the company explains, Diet Coke was aimed at "baby boomers who were getting 20 years older and 20 pounds heavier."

The bulk of its strength is still in that group, which is a long-term problem. Consumer product companies are wary of being tied too closely to the current oldest generation because no group of people lives forever.. At the same time, the drink still has a 26.3 percent market share of dollars spent on diet soda. Diet Pepsi has about half that much.

Coca-Cola clearly wants to attract millenniums to the brand and expand beyond its historical base. That explains a dual set of can designs and additional flavors.

Coca-Cola used a 26-year-old designer who was a long-time Diet Coke customer that presumably would have insight into what might appeal.

The new slimmer cans will be found in and among shorter and fatter ones. The contrast becomes an unconscious psychological signal that drinking Diet Coke will make you thinner. The oldest millennials, in their mid-30s, are starting to experience the frequent older-and-heavier reality of aging. This is an attempt to head off the gradual decline of the main customer base by bringing a new one into the fold.

At the same time, the company will keep the original packaging as well. Too much emphasis on the new could be off-putting for that critical core customer group.

New flavors were a result of 10,000 people in focus groups over two years looking at 30 plus potential flavor combinations. The new tastes (be thankful that lemon lavender tea didn't make the cut) are also intended to appeal to millennials but aren't a replacement for the basic formula. Coke learned decades ago with New Coke the ill-advised nature of completely changing a drink's formulation. It's more adding an additional flavor to the cola.

Expect to see celebrities and social media influencers tied into the brand, but not top entertainers and pop stars. Coca-Cola wants to gain from the association but not put the product into someone's shadow.

If you're facing your own need to rebrand, here are some lessons to keep in mind:

  • Find a way to add the branding rather than completely pushing out what people expected. The latter can mean losing a significant customer base, as people either get irritated or overlook the new packaging and assume they can't get what they had wanted.
  • Do your research to see what people might want and test different versions. It's not foolproof; one of the least reliable types of market research is to ask consumers what they would do under hypothetical circumstances.
  • Recognize that you might need to bring in new creative help to get the right insight and tone.
  • Don't be slow, but do be deliberate. If you screw up a rebranding, your entire business will take a hit that won't stop hurting for a long time.