The witching hour yesterday came at noon when the 98th Congress convened and lame-duck lawmakers like Virginia's William C. Wampler officially ceased to be members of the United States House of Representatives.

"Yeah, at 12 o'clock," said Wampler's chief aide, Steven Berry, "I turn into a weasel, the girls in the outer office become pumpkins, and poof, it's like we never even existed."

For Berry and the little coterie of aides who joined him yesterday morning in the barren office, the overriding feeling was one of expendability, as nearly everything that once was Wampler's--the typewriters, computer system, even the big cutout map of the district Wampler had represented for 16 years--was moved out and turned over to the man who beat him in the November election, Democrat Frederick C. Boucher.

"You'd like to think there'd be some kind of void from the loss of a member or staff," Berry mused. "But things just go right on. They never miss a lick."

Yesterday, although Wampler did not make an appearance on Capitol Hill, one vestige of his tenure remained. When constituents or friends of Boucher dialed what they thought was the new representative's number, they wound up with Wampler's old office.

"Congressman Wampler's office," Suzanne Doggett, Wampler's legislative aide, answered sweetly each time the telephone bell jangled. "Yes, we're still hanging in here," she told one friendly caller. "No, the number will be switched to Mr. Boucher some time this afternoon," she told other befuddled callers who mumbled apologies when they realized they had reached the man Boucher had ousted after a difficult campaign, a hairbreadth victory and a recount demanded by Wampler.

For the convenience of constituents of the 9th District in southwestern Virginia, Wampler's congressional telephone number gets switched over to the new member.

But Boucher does not get the airy office in the Rayburn Building with its fabulous view of the Capitol dome that Wampler acquired through years of seniority. Instead Boucher moved into an office on the seventh floor of what is considered the less desirable Longworth Building. Capitol Hill telephone installers jammed up with scores of orders for new congressional offices were unable to complete all of them yesterday.

The lapse in phone service did not seem to bother Boucher, who happily joined his colleagues in a mass ceremony in the House chamber and swore to "well and faithfully discharge the duties" of his office.

After that he raced to one of the cavernous committee rooms in the Rayburn Building to accept the hugs and handshakes of several hundred supporters, some of whom had traveled by bus from the district to celebrate with him.

He also took his first legislative step as a congressman--already distancing himself from his conservative Republican predecessor. Boucher joined 221 colleagues in sponsoring a new resolution for the Equal Rights Amendment. Four years ago, Wampler voted against giving the states more time to ratify the amendment.