Author Jeff Kinney saw the first signs of unrest after a test screening of his fourth film, “Diary of a Wimpy Kid: The Long Haul” (now in theaters).

The road-trip family vacation comedy with the entirely new cast playing the Heffley family — Alicia Silverstone (as mother Susan), Tom Everett Scott (father Frank), Jason Drucker (title protagonist Greg) and Charlie Wright (brother Rodrick) — went down well. But there were signs of a burgeoning “uh-oh” situation.

“We got a lot of laughs,” says Kinney, an executive producer on the film. “Then the lights came on and people started asking, ‘Why didn’t they use the old cast?’ They were genuinely confused about why we would replace such a good cast. It took us a little bit off guard.”

Fans of Kinney’s books exploring middle-school life had become accustomed to the cast of the previous three films, starting with 2010’s “Diary of a Wimpy Kid,” including Rachael Harris (as Susan Heffley), Steve Zahn (Frank), Zachary Gordon (Greg) and Devon Bostick (Rodrick).

When the first “Long Haul” trailer showing the new cast arrived online in February, the Generation Z audience made its voice heard on social media, focusing around flashpoint character Rodrick with the #NotMyRodrick hashtag (more than 270,000 instances on Instagram alone.)

“It was this nostalgia and displeasure at seeing the old cast gone,” says Tara McNamara, entertainment correspondent for the parental site SocialMoms. “I don’t think we knew before what films really connected with Generation Z, but we just found out: It was ‘Diary of a Wimpy Kid.’ Who knew?”

For Kinney, the decision to change the cast was simple and inevitable for the franchise focused around middle school.

“The ‘Wimpy Kid’ movies are different from ‘Harry Potter,’ which follows the kids as they age,” says Kinney, adding that main character “Greg is frozen in this preadolescent amber while young actors grow and change. This kind of movie really begs to be re-invented every few years.”

Gordon, now 19, had outgrown the “Wimpy” Greg character he started playing at 11. Bostick, now 25, was out of the age range of Greg’s older brother, a high schooler.

Silverstone, who played Batgirl in 1997’s “Batman & Robin”, compares it to the changing of superheroes. “It was the same with Batman. People get attached,” she says. “But you cannot continue to play a ‘wimpy’ kid when you have muscles and a deep voice. That’s a different story.”

McNamara says her 16-year-old daughter has been a fan of the “Wimpy” books and movies, and was disappointed to find out about the cast change. #NotMyRodrick came about because Bostick had been “the crush-worthy one of the cast,” McNamara says.

Her daughter isn’t likely see “The Long Haul,” but McNamara’s 6-year-old son has no history with the past films and is a fan of the new one after attending a prerelease screening. Its “going to be his movie of the summer,” says McNamara, who doesn’t envision the outcry hurting the movie’s box office or home release.

Kinney believes audiences will come around. “People are still so invested,” he says. “It’s going to take kids a few minutes to adjust to the new cast and then we’re off to the races.”

Bryan Alexander, USA TODAY