Aug 31 will go down in network television history as the night "Happy Days" went sad.

A celebrity-packed audience watched with tears in its eyes as the marshmallow show about the '50s began tackling the heavy, uneasy issues of the '60s.

The word already had gotten out on the Aramount TV lot, but the full impact didn't sink in until three minutes into the show.

It caught The Fonz (Henry Winkler) and the audience in mid-sentence as the cameras were running on episode six of the new season (to show in mid-October).

Fonzie was hunched over a slightly battered '55 station wagon when the guest star for the night came onto the set. The actor was young, good-looking and glib, with the same kind of open face as the show's regulars Ron Howard and Anson Williams.

And he was in a wheelchair -- paralyzed from the waist down.

"I'm here for the job," said the actor, a former Olympic-class pole vaulter named Jim Knaub. Fonzie's back was still turned, and the audience had gone silent -- a rare phenomenon on the "Happy Days" set.

"Be right with you," said Winkler. "Hand me that oil filter on the top shelf, will you?"

"No, snarled the new character, reaching vainly for a shelf that was two feet above his chair. Fonzie whirled around and froze: "Oh."

"Don't worry. It's not contagious," said Knaub, playing a bitter paraplegic martyr named Don.

From that line on-- until only seconds before the end of the show -- "Happy Days" plunged darkly out of the sock-hop genre into a heavy script by Fred Fox Jr. that tackled the issue of being handicapped in a walking world.

Like "All in the Family," "Maude" and "The Mary Tyler Moore Show" before it, the 7-year-old sitcom had begun to use its comedy to enliven a mainly sober message. But unlike those three, the regulars on "Happy Days" (Richie, Fonzie, Potsie and the Cunningham family) always had been locked into soda-fountain issues: the price of a hamburger; the outcome of Richie's basketball game; Fonzie's hottest date; Joanies new figure. These have been the staples of television's most highly rated show (on a regular basis) since 1972.

"It's new territory. And it's heavy territory," said producer Garry Marshall -- creator also of "Laverne and Shirley," "Mork and Mindy" and "Angie," and in terms of rating points, the most successful TV producer of the '70s. "But we are committed to it. This is a semi -permanent change, not an asolated show."

Mid-Life Crisis: As altruistic as the "Happy Days" trend may seem, it is hardly a gift from Paramount television to American social awareness. The show has had some ratings problems, and the script changes are expected to raise Nielsen points as well as consciousness.

An extensive new study by veteran television consultant Michael H. Dann of New York shows that sitcoms, like the average American, go through middle age and then hit a sort of audience mid-life crisis.

And if plots don't start jarring the faithful but gradually aging viewers -- with scripts that reflect their changing, more mature attitudes -- the shows' days are numbered.

Dann, whose Television Consultants Inc. advises more than 20 organizations (including Arner Communications), says that "the history of a successful sitcom shows a remarkable program loyalty. The millions that started watching 'Happy Days' in 1972 will grow up and want more from their viewing. The audiences that liked 'All in the Family' 10 years ago are the same audiences that are watching it today. But it is a much gutsier show."

The truth of this came home to ABC last season when "Happy Days" began losing its toehold on the top of the ratings to the same network's "Laverne and Shirley" and "Mork and Mindy."

Marshall says, "It is interesting to watch the shows go up and down. But if you don't watch out, one of them slips and stays there. A show can start repeating plots over and over."

"Laverne and Shirley" is an example: "We are working on it now because it has tended to fit the old 'I Love Lucy' mold," Marshall says.

If seriousness can spell success, it is no surprise that the trend is growing. Last week, "DiffRent Strokes" took on the topic of bigotry, and in October "Hello, Larry" begins dealing with divorce.

And television's newest production showcase company, the John Charles Walter group, has leaped into the new season with heavy themes in both "Taxi" and "The Associates." They have plots that run halfway between Greek tragedy and Neil Simon. "The producers knew they wanted to be heavy with social overtones from the beginning," says Dick Winters of Paramount Television.

For example, two episodes of "Taxi" will examine the problems of an excessively overweight girl dating a sexy man. The first episode has Judd Hirsch dating the fat girl and trying to adjust as she tries to take the chip off her shoulder. The second show brings the same situation back on to the series: This time she is 60 pounds lighter. "But the problems the characters face are even worse," says a spokesman for Paramount.

"Another factor for change is guilt," Dann says. "Once a show becomes successful, producers often feel a little bit guilty about their success. They have told me they simply have to give something back."

But two of the shows in last year's top 10 will not change from their whipped-cream formats. A spokesman for the producers of "Three's Company" and "The Ropers" says that those shows, still early in their runs, will not delve into deeper issues.

Alas, says Dann, there are some shows that simply have nothing to give.

"Charlie's Angels", he says, would go off the air in 15 minutes if it went heavy. The same goes for 'The Love Boat' and 'Fantasy Island." Producers on these shows would be fired the day after the first heavy episode was shown."

Dann believes that the sound could be turned off "Charlie's Angels" and it would make not the slightest bit of difference to the show's rating.

But the changes hitting "Happy Days" and other programs -- referred to on the West Coast as "Learism" after producer Norman Lear -- could backfire, according to Television Consultants Inc. Dann cites the example of "Baretta," starring Robert Blake, which switched to serious subjects after several successful years. "The early shows were clearly melodramas and let Robert Blake spend a lot of time in disguise," Dann said. "That made the show different. That was its gimmick. When Blake pushed the social issues into the show, it was reflected immediately in the ratings. I think it died earlier because of that."

Dann warns that "Norman Lear is very gifted in dealing with real problems while keeping his shows funny at the same time. He has that special balance. This is a very very tough thing to do and it is a real gamble."

Although the "learizing" of shows like "Happy Days" and "Laverne and Shirley" can gain time and rating points, Dann says that shows can become too heavy for the network "establishment."

"A cause celebre developed on 'The Dick Van Dyke Show' -- the later show with Hope Lange -- where an episode dealing with parents in bed was pulled off the air," Dann recalls. "Carl Reiner, the producer, told me he wanted one of the children to find the parents in bed together. This is a situation that affects almost every family.

"The film was eventually made but was never shown. CBS decided that America was not ready for it. I can forsee things like this happening all over again as "'Happy Days' and 'Taxi' head out into deeper water."

Getting Attention: "We could be taking a chance, but I don't think so," says producer Marshall about the new "Happy Days" emphasis. "It is time for this show to stretch its wings and move into the uneasy years of the '60s. We're going to take on the little things like longer hair, the appearance of the first hippies and the disappearance of the happy innocence of the '50s."

The music will change from the Big Bopper to The Beatles.

"Ron Howard as 20-year-old Richie is going to fall for an older women. Fonzie will become heavily involved with a deaf girl. Marion and Howard will need to reaffirm their marriage. None of this is as easy to do as it sounds.

"We just can't sit still and not use the enormous power that this show has achieved, and we can't get frozen in the '50s."

But several officials at Paramount Television have said that there are also other reasons for the script changes. "Marshall and a lot of other people connected with the show are sick of the show's achievements being dismissed by the Academy of Television Arts and Sciences and other groups," said one executive. "It is routinely passed over for Emmys and other awards in spite of the level of its acting. No show has been ignored like this since the early days of television."

(To make the point, the executive points out that Ron Howard, in spite of two Golden Globe Awards, has never even been nominated for an Emmy.)

The new themes apparent in the Aug. 31 filming could make the difference.

In the Fox script (called "The Mechanic"), attitudes toward the handicapped are examined from both sides of the wheelchair. The paraplegic Don remains antagonistic until the final seconds of the show. "He has a chip on his shoulder as wide as a two-by-four," says Fonzie to Richie.

Fox, 32, a scriptwriter on the show for more than two years, researched his idea for weeks before starting to write. "I tried to find out not only what the problems were, but what the handicapped people were feeling," he says. "I tried to enliven each heavy point by some typical 'Happy Days' humor."

Perhaps the most effective scene shows Fonzie getting his own wheelchair and confronting Don at his level. But when Don closes in on him, Fonzie leaps out of the chair and doubles up his fist. "That's not fair," says Ron Howard as Richie, "You're out of your chair. You lost. You're standing. Fonz, you were never equal to Don this way. You can always get up from that chair. Don can't."

Marshall says he was determined to underscore the point by casting Jim Knaub, a real paraplegic, in the part. "We hope to make him a regular, and he's turned out so well I am thinking of creating a show around him. He's a highly appealing actor."

The tension at the show's run-through on Wednesday was apparent to everybody. Marshall, his assistants and the entire nine-member staff of writers were on hand -- with the writers often changing lines at a word from Marshall or director Jerry Paris. Marshall says a lot of the tension came from the "obligation we have to the 50 million or more people who will see this episode. Our characters have identities and powers all their own, and a couple of lines from Richie or Fonzie can alter the thinking of millions of kids."

(This was borne out last season by an episode called "Fonzie Gets His Library Card." After the show -- where Fonzie told how important it was to read -- the American Library Association reported that the number of library cards among kids 9 to 14 increased 500 percent.)

If the new formula works, Marshall says he will spread the idealism, to "Laverne and Shirley" and ,Mork and Mindy" -- in many ways lighter shows than "Happy Days."

"We're already thinking about confronting the assassination of President Kennedy and the aftermath," says Marshall. "But there's a lot of work to do before that."