Geared Up for Thursday Night’s Family Feud : Television: The face-off between ‘Simpsons’ and ‘Cosby’ heats up as the latter offers its first new episode of the season tonight.

It holds elements of an epic confrontation: “The Cosby Show,” aging and hinting vulnerability, challenged head-on by the smart-alecky pop culture phenom, “The Simpsons.”

Just the thought--can the great Cosby really be taken by a cartoon?--seems impossibly brash, but then the bold stroke is the signature of the Fox network, which posed “The Simpsons” gambit early last summer.

After four weeks of dueling with reruns, the real battle--at least half of it--begins at 8 tonight when NBC debuts the seventh season of “The Cosby Show,” which spent its hiatus undergoing an overhaul. It will be three more weeks before “The Simpsons” is back with new episodes.

The “Cosby” changes, as well as the challenge from Fox, were prompted by signs last season that the show was teetering at the top. Where viewers had once religiously given “Cosby” TV’s highest ratings for a series, the show had lost its lock-grip on the No. 1 spot and started to show signs of ratings mortality, finishing behind ABC’s “Roseanne.”

But more, the show seemed tired and a bit stale, relying on such script gimmicks as dream sequences, and the spreading intuition in the industry was that “Cosby” had reached the point that all mega-hit TV shows eventually come to: held at the top by viewer routine, but poised for a fall.

Bill Cosby and the show’s executive producers, Marcy Carsey and Tom Werner, sensed that their show needed revitalizing, and “Cosby” got a new executive producer to supervise production in New York, the writing staff was swept out and nine new writers were brought in. And a new character was added to the Huxtable household--Pam, a 17-year-old relative of Clair Huxtable’s with a disadvantaged background.

There will also be changes in the existing characters, reflecting the fact that five of the new writers are women: Clair’s career as a lawyer will become more substantial and demanding, Vanessa will start high school and Denise, a college drop-out, finally gets her life in order.

“The changes are helpful,” says Bernie Kukoff, the new executive producer, “because they add texture.”

The possibility that “Cosby” was vulnerable resonated deeply, because an omnipotent “Cosby” has been the foundation underlying network programming strategies for the last several years. Thursday night became “Cosby’s” night, which made it NBC’s night, a fact that rival programmers’ conceded by filling their Thursdays with programs bearing low ratings expectations. Until last week, NBC had not lost a Thursday night since February, 1988.

In challenging this daunting force, Fox is gambling with its most valuable property. “The Simpsons,” a sardonic animated view of family life that is the antithesis of “Cosby’s” warmth, is more than just a popular cartoon. It’s a genuine phenomenon in the mass culture, inspiring Simpsons’ T-shirts, Simpsons’ lunch pails and, most pervasively, a Simpsons’ attitude.

More significantly for Fox, the show was the network’s first series to hit the rarefied reaches of the Top 10. Fox hoped that juice would transfer to Thursday, where “The Simpson’s” would become the linchpin of a new night of programming for the network.

It was a nervy step, admired even by the competition. “I really admire Fox,” says Marcy Carsey, co-executive producer of “Cosby.” “I think it was a wonderful move on their part.”

The early results seem to suggest otherwise. In its Sunday night time slot, “The Simpsons” reached No. 4 in the Nielsen ratings. But in August, when Fox moved the show to Thursdays, “The Simpsons” sank as low as No. 54. It was No. 45 last week.

This was only a contest between reruns, a summertime prelude to the real battle this fall, but the sobering result has Sam Simon, co-executive producer of “The Simpsons,” wondering about Fox’s strategy.

“The gallows humor started immediately” says Simon, describing the mood of the “Simpsons” crew. “We were nervous and unhappy about the move (from Sundays). The numbers thus far have made us even more nervous.”

Fox officials have stoutly defended the move, insisting that the ratings plunge is not as bad as it looks. They point to the valuable demographics of their show, arguing that “The Simpsons” is actually doing just fine among the younger audience that advertisers prize--especially considering that the network has fewer affiliates than NBC and that the show is in its third cycle of broadcasts.

Fox’s hope is that ratings will improve when new episodes of “The Simpsons” begin Oct. 11. Producer Simon, meanwhile, is more anxious.

“ ‘The Cosby Show’ is a quality, classic show that has a franchise on Thursday night,” he says. “When I heard that ‘Cosby’ was ripe for the taking, my mind first flashed on how many shows have tried to take on Johnny Carson, who was supposed to be ripe for the taking.”

Simon says that “The Simpsons” wasn’t expected to topple “Cosby” right away when the show was moved last month, but that Fox had predicted it would fare better than it has. Research indicates that “The Simpsons” lost some child viewers and some of its female audience with the move.

Simon’s deeper fear is not simply that his show will lose the battle of Thursday night, but that the war against “Cosby” may quicken the life cycle of “The Simpsons,” which went on the air as a series only last January. He has the sensation of watching his show’s life flashing before him.

The fast rise of “The Simpsons” as a pop culture phenomenon was fueled at least partly by its perception as a hot vehicle. But what perceptions bring up, perceptions can also bring down, and that is what worries Simon.

“The whole ‘Simpsons’ phenomenon has been so compressed,” he says. “We became a ‘critics favorite,’ then the next week we were a ‘cult hit,’ the next we were a ‘ratings hit,’ the next we were a ‘threat to society.’ And the next, we were ‘over-merchandized.’ And that (cycle) usually takes two years.”

For now, Simon can only wait for his show to begin airing original episodes, and hope that new shows will fetch more viewers. If not, he wonders whether Fox will consider moving his show back to Sundays, and even if it does, whether it will be too late.

“The history of television,” Simon says, “is not the history of networks admitting they made a mistake.”