Longevity on TV derives from two things: a winning original concept and invigorating change that over time modifies — but does not elementally alter — the initial formula.

That’s certainly the trajectory that “Wheel of Fortune” has followed over the past quarter century, and credit for that success rests with two men, the late Merv Griffin, who created the show in 1975, and Harry Friedman, who joined the show in 1995, becoming its executive producer four years later.

Karen Griffith, now one of “Wheel’s” two supervising producers, worked on the show with Griffin before it was syndicated, when it aired daytimes with Chuck Woolery and Susan Stafford as hosts. She recalls her former boss, who died in August, as very hands-on.

“Up until the end of his work on the show,” Griffith says, “Merv knew every puzzle before it went on the air. He would meet with our producer and approve all the packets. Sometimes, he wrote puzzles himself, (even) on cocktail napkins on airplanes, and then he’d hand the producer a bunch of them. He was very into the show.”

But Griffin’s involvement with “Wheel,” essential ceased years ago. After he sold Merv Griffin Enterprises to Columbia (then owned by Coca-Cola) in 1986, his interests shifted to real estate. And by the time Sony acquired “Wheel,” when it purchased Col three years later, Griffin’s connection was purely historical.

Friedman arrived under Sony’s stewardship, when the show needed refreshing.

“One conclusion I came to early on is change the show but don’t change the game,” he recalls. “And I still believe that. The show is virtually the same as when it started on daytimes, minus the shopping.”

His improvements have been both varied and ceaseless. “Wheel” has a different theme and look each week, and the show takes regular “road trips” to places like Hawaii. A recent remote from Radio City Music Hall in New York marked the show’s 52nd off-site location.

Using electronic screens instead of light boxes for the puzzle letters was one of Friedman’s earliest innovations.

“Vanna used to turned the letters around by hand,” says Friedman. “It was very quaint, but quaint became not the direction we wanted to go in. That was 1996.”

Puzzle content has also evolved. “For a long time it was just person, place and thing,” says Friedman. “Now we have categories like ‘On the Map’ and ‘Food and Drink.’ They reflect more active lifestyles. And we’ve introduced a category called ‘What Are You Doing?’ For that, the puzzle might be ‘Running along the beach.'”

There’s also the Wheelmobile, a 32-foot-long, bright-yellow Winnebago that travels to roughly 20 cities a year in search of new contestants. Friedman first road-tested the concept eight years ago.

“The Wheelmobile helps us find the most energetic contestants, which helps make for a better show,” he says.

The success of the 4-year-old Wheel Watchers Club, TV’s first permanent online viewer-loyalty program, suggests that innovations are unlikely to end anytime soon.

“I think there’s no limit to the growth potential of ‘Wheel of Fortune,'” Friedman says, “so long as we keep asking, ‘What if?'”