Old Soap Characters Never Die--Sometimes They Change Shows : Television: Creators often look to crisscross into other series if low ratings and waning story lines need a boost.

In a recent story line on CBS’ top-rated daytime soap, “The Young and the Restless,” nurse Sheila Carter Granger suffered a miscarriage but faked a full-term pregnancy, then swapped the newborn child of her husband and his ex-wife with one she had bought, passing off the ex-wife’s baby as her own.

Now Sheila is carrying off an even more ambitious switch. One day after she last appeared on “Y&R;,” where she had been presumed killed in a fire, she turned up on “Y&R;’s” sister serial, “The Bold and the Beautiful”--and is caring for the young son of lead “B&B;” couple Eric and Brooke Forrester.

“It’s neat to get to be the same person, get to do the same nasty things,” says Kimberlin Brown, the actress who plays Sheila. “Obviously, they can’t have me be as bad as I was in (“Y&R;’s”) Genoa City because I’m trying to make a better life for myself. But (the audience) knows what Sheila’s capable of.”

The unusual move, a first in the two shows’ histories, was inspired by the reluctance of their co-creator, co-executive producer and head writer, William J. Bell, to terminate the Sheila character despite her waning “Y&R;” story line.

“I had found Sheila to be a very provocative character,” he says. “She was damn interesting, in the way her mind worked. I knew this was a classic character that the audience loved to hate. As her story was winding down, there was not a day that I didn’t have great regrets that I was going to lose her.”

Once he hit upon the idea of a soap switcheroo, Bell embarked upon a two-month series of behind-the-scenes machinations to keep it a surprise.

Creative staff and network executives were informed of the development on a need-to-know basis. Brown was sworn to secrecy, allowed only to tell her husband and agent. The pages of the “Y&R;” script that contained the revelation that Sheila was still alive weren’t distributed until the last minute.

“It drove everyone nuts. They were beside themselves,” says Brown, who had resigned herself to being written off “Y&R;” and was therefore thrilled at Bell’s proposal. “The cast was pumping me left and right.

“It was the hardest thing in my life, having that utmost secrecy,” she says. “The most exciting day was when I could open my mouth and tell everyone. They seemed extremely happy for me.”

Though Sheila’s move is unique among the Bell soaps, she is not the first daytime serial figure to crossover from one show to another. In 1970, when NBC’s “Another World” was divided into two half-hour shows--”Another World--Bay City” and “Another World--Somerset”--the latter used three characters from the original series.

Six years later, “Somerset’s” replacement, “Lovers and Friends,” also borrowed characters from “Another World.” And in 1980, “Another World” villain Iris Carrington (soap veteran Beverlee McKinsey) moved to the show’s spinoff, “Texas.”

Over on ABC, characters on the two shows created by the venerable Agnes Nixon--”One Life to Live” and “All My Children”--have occasionally gone back and forth from their respective East Coast locales of Llanview and Pine Valley, according to Mary Alice Dwyer Dobbin, senior vice president of daytime programming. The same psychiatrist counseled characters on both, and one of Erica Kane’s (Susan Lucci) many wedding gowns on “AMC” was created by an “OLTL” fashion designer.

More recently, “AMC’s” dastardly Adam Chandler (David Canary) and “OLTL” tabloid editor Dorian Lord (Elaine Princi) have been seen conniving together by phone. And during last November’s sweeps, in a move to increase viewership for ABC’s ratings-poor “Loving,” “AMC’s” popular Ceara Connor (Genie Francis) and Jeremy Hunter (Jean LeClerc), turned up in the “Loving” town of Corinth; Jeremy even spent a day in “OLTL’s” Llanview en route. When the two returned to “AMC,” Corinth residents attended their wedding.