Transporting the entire Washington staff of the largest public relations firm in the world to new quarters may sound like an onerous task. But compared with some of the other challenges encountered recently by Hill and Knowlton's local outpost, the disruption in the move to new offices on Pennsylvania Avenue NW can only seem a minor irritation.

First, Hill and Knowlton was acquired by JWT J. Walter Thompson Group, one of the world's largest advertising agencies, in a merger designed in part to build capital for Hill and Knowlton and to help expand its operations, particularly in Latin America and Canada.

Then there was the departure of vice chairman Bob Gray, the firm's prime mover and Republican Party insider. When Gray set up his own shop in Georgetown, he took a chunk of Hill and Knowlton's staff with him, and speculation was that a bevy of the firm's clients would follow. Gray himself launched a full-court press on his former coworkers, and spoke unequivocably of becoming "the best and biggest PR firm in the United States."

Gray was replaced at Hill and Knowlton by Bob John Robison, who had become Washington vice president of the FMC Corp., a manufacturer, after establishing Hill and Knowlton's offices in Dallas and Houston.

Now the firm has moved to what one staffer terms its "bodacious new digs," chosen by Gray before his departure, in a plush glass and concrete high-rise at 1201 Pennsylvania Ave. Other tenants in the still-unfinished building include Covington and Burling, Hughes, Hubbard and Reed, Rolls Royce, Control Data Corp., Hallmark Cards Inc. and Joseph Seagram and Sons. Last week, the spacious, high-tech offices were the scene for a lavish housewarming attended by about 700 persons.

While Robison concedes that Gray's departure did make some waves at Hill and Knowlton, he adds that the operation has been streamlined and business has doubled.

"Sure, there was some disruption when Bob left," Robison said. "After all, he'd been here 20 years. For the first few months it was a rocky time.

"Some people left but that wasn't all bad, because the business base when I arrived wasn't there to support the number of people that were here . . . Some clients left but not major ones. We've replaced all the business and more that was lost" when Gray left.

"In fact," he added, "our business has increased more than 50 percent from a year ago."

Some personnel changes have come on the heels of Robison's appointment, with the Washington staff now at 40. Internationally, Hill and Knowlton employs about 1,100 people, which makes it "the first PR unit to break the 1,000 mark in staffers," according to the 1982 O'Dwyer's Directory of Public Relations Firms.

O'Dwyer's also says Hill and Knowlton had a 30 percent increase in fees in 1981, to $46 million, with about $3 million to $4 million of the $12 million gain resulting from adding the staff and accounts of J. Walter Thompson's New York, London and Frankfurt offices.

Hill and Knowlton Chairman and Chief Executive Officer Loet Velmans, who was in town for the party at the firm's new offices, said that business so far this year seems to be unaffected by current recessionary pressures and budget tightening.

"I can't forecast for this year, but the year has started off with a bang," Velmans said. "The type of public relations assistance that Hill and Knowlton is being asked to provide is not really related to the economic cycle of recession and recovery. That's based on our long experience....We will continue to expand because we will continue to open new offices, which will add to our revenues, and we will continue to diversify in new services, as we have every year in the last three to four years."

Velmans stressed that the acquisition by the JWT Group has not been the major factor in any recent Hill and Knowlton success.

"We continue to stress the separateness of the disciplines of advertising and public relations ," Velmans said. "It's not that we have now an influx of J. Walter Thompson business. We get our clients on our own as we've always done . . . We are completely independent and it has really not changed.

"We've had some benefits in terms of help that they've given us in looking at new areas outside of this country, in Latin America and Canada, where they'd been strong and we were not at all."

One new service Hill and Knowlton has organized is a state and local government relations service tailored to address clients' needs under the Reagan administration's New Federalism program. Velmans said the firm is among the first to offer a system under which it has contacts in the governments of each of the 50 states.