We were paying for a few things at the Hechinger's in Montgomery Mall the other night when I suddenly grasped my wife's shoulder for support.

"I don't believe it," I cried. "Look! Up there on the wall! Harriet Homeowner has a new hairdo!"

I felt a little like I did the day that DUpont 7 became 387 at the hands of the phone company. Another pillar of familiarity gone. Another perfectly good mascot gone madly mod.

Ever since creation, it seemed, the female symbol of Hechinger's had been the paragon of cheer and activity. Harriet was always tilling the soil or painting a wall, her eyes expectant, her smile shining.

But the key feature of her persona was always her scarf -- carefully wrapped around a head of hair that looked long but whose true style was always shrouded.

And now, up there on the wall, not only was the scarf gone, but in its place was . . . a shag!

"She looks like she's wearing a bathing cap with creepy crawlies on it," I said to Jane. "I think she looks crummy." But Jane was three steps ahead of me as usual. She had been leafing through a stack of suggestion forms, and she presented me with one.

"I don't think you're ready for this," she said.

I wasn't. There was Harriet again -- in yet another new style. This one was a short, swept-wing number in which Harriet's ears were covered with lighter patches of hair. Could Harriet have become a peroxide user? What's going on here, Hechinger's?

"We've always tried to make Harriet such that women can identify with her," said Pam Quigley, a Hechinger's public relations operative who doubles as the staff archivist. "So lately we've started changing her hairdo from time to time.

"Your wife doesn't wear her hair the same way all the time, does she? Well, neither does Harriet."

Interestingly, though, Harriet's spouse Harry hasn't changed a bit in the last 13 years, Pam said. Through Vietnam, through Watergate, through gobs of inflation, Harry has been the same resolutely dowdy guy, his sports shirt open at the neck, his sleeves rolled up, his porkpie hat worn right in the middle of his head.

Harry first appeared in Hechinger's ads in the early '50s. For the next decade and a half, he changed regularly, almost furiously. Beginning as a boyish imp, he became, in order, a less boyish imp, a doll in western garb, a doll in jeans and finally the Mr. Average we know today.

Harriet debuted on March 2, 1975. Until just a few months ago, she was a symphony of subtlety and solidity in her scarf. But in the immediate future, according to Pam, her hairstyle will "probably change, maybe even from week to week. It depends on the fashion. We want her to be chic."

I trembled. Just what form might "chic" take in the months ahead?

"Can I tell you a secret?" Pam asked. "In a couple of weeks, she'll be getting a 'do' we modeled after Lady Di's. Everyone is wearing it now, and Harriet should be no exception."

Well, fair enough, I guess. It's just hard for me to imagine a Diana-like character mixing cement. After all, the real Di is on record as saying she'll never make breakfast for Charles.