“This was absolutely brilliant of them to change it and not say anything,” said Lynn Dornblaser, director of innovation and insight at the market research firm Mintel.

Now Kraft is getting a little more vocal.

A new ad campaign plays up the element of surprise involved with the company’s new mac and cheese formula, with 15- and 30-second broadcast and online video spots featuring the former late-night television host Craig Kilborn and the tag line, “It’s changed. But it hasn’t.”

Kraft was concerned that people would perceive a change in flavor that wasn’t really there if it made too big a deal about the different formula as soon as it started using it. In fact, when the company made the announcement last spring that it would be tweaking the ingredients, Mr. Guidotti said, people began posting on social media their concerns that the mac and cheese would taste different.

“We knew we wanted to address that tension,” he said.

Some on social media even said, shortly after the April announcement, that they thought the mac and cheese tasted different when, in reality, they were still eating the previous version. This is a psychological quirk, well known to food manufacturers, that can stymie well-meaning attempts to make processed foods healthier.

“Anytime there’s a suggestion of what something should taste like, some aspect of taste, when we try that food, we’re looking for it,” said David Just, a professor of behavioral economics at Cornell University who is affiliated with the Cornell Food and Brand Lab. “Whenever you have labels like ‘healthier’ or ‘reformulated,’ people are looking for the absence of a taste they really like.”