After almost a century in its downtown location at 624 Indiana Ave., the Central Union Mission, a retreat for the homeless and downtrodden, is moving to a vacant automobile showroom at 14th and R streets NW.

As workmen begin converting the old showroom to a residential facility, the sounds of construction become another reminder of the gradual transformation of 14th Street, still suffering from the 1968 riots and many years of prostitution and drug trafficking.

Much of the transformation along 14th Street is occurring between Thomas Circle and U Street NW. At One Thomas Circle, the Prudential Insurance Company is building a 12-story office building with retail store space on the ground floor and 225,000 square feet of office space above. Adjacent to the Prudential building is the Vista International Hotel owned by Hilton International. The hotel, not affiliated with the Hilton Hotel chain, will have 400 rooms and is expected to open in early 1983.

Edward Jaffe, leasing representative for One Thomas Circle, said Prudential decided to build its office building there because "It's a great location, there's a lot of development in the area."

North of Thomas Circle, at 14th and U streets NW, the city has proposed building a government services office building near the proposed Metro Green Line subway stop. City officials hope that the government building will help stabilize the neighborhood and attract new businesses.

Property on 14th Street between U Street and Thomas Circle has been changing owners for several years. In March, for example, New World Communications Inc. (NWCI), an affiliate of the Rev. Sun Myung Moon's Unification Church and owner of The Washington Times newspaper, bought all but two lots on the west side of the 1500 block of 14th Street. The company bought 1502-06, which houses Dickey's Hardware, for $350,000 and an empty warehouse at 1510-20 for $950,000. Only a corner parking lot and Sam's Pawn shop, on either side of Dickey's, were not bought. In April, NWCI purchased another building at 1337 14th St NW that houses the Northwest Office Supply Co. for $225,000.

NWCI spokesman Larry Moffitt said the buildings were purchased initially to house the editorial offices of The Washington Times before the newspaper's New York Avenue facilities were acquired. Moffitt said plans for the future use of the buildings are unclear.

Real estate brokers say the 14th Street strip has held the speculative interest of developers since the downtown office boom began to fill Washington's skyline with construction cranes several years ago.

And homeowners, willing to pioneer in sometimes-tough neighborhoods, have continued to renovate the turn-of-the-century and earlier Victorian homes, and to band together in organizations such as the Logan Circle Community Association to fight prostitution, drug trafficking and other crime.

Residents have testified against prostitutes in court in an attempt to secure stiffer penalties, and police have responded to citizen pressure by maintaining a high presence and by setting up barricades and roadblocks along the 14th Street corridor to confound the prostitutes' late-night customers as well as potential drug customers.

Their efforts to improve the strip and the surrounding neighborhood have made it more attractive. Police said crime in the area has been reduced 30 percent recently.

In the midst of this neighborhood metamorphosis, the Central Union Mission is expected to work a change of its own among neighborhood transients and others who gather at street corners and at the service station at 14th and Corcoran NW.

The mission vacated its six-story Indiana Avenue NW structure when the Pennsylvania Avenue Development Corporation bought the space for commercial and residential development, a part of Pennsylvania Avenue's urban facelift.

The mission's new 14th Street facility will temporarily house 90 men in its male transient quarters and 15 women and 20 men in its outpatient alcohol recovery program.

When word of the mission's relocation was announced, the Rev. Robert R. Rich, the mission's executive director, said the community was concerned that the facility would draw more derelicts to the area.

"We just had to meet with them the Logan Circle Community Association and explain our programs," he said, emphasizing that the mission's role is to get people off the street by caring for their physical and spiritual needs.

"But one thing's for sure," he said, noting the goal the mission shares with LCCA, "we're definitely out to change the face of 14th Street."

"I've seen this neighborhood change," said Reuben Underwood, branch manager of Mattos Automotive Paint Store at 1811 14th St NW. "I've seen a lot of residents displaced. I've seen a lot of old residents forced out of homes because of rising income taxes, utility bills and redevelopment."

Near 14th and T streets, playing cards still cover the table set up in front of a boarded-up business, as old-timers in Greenline and Adidas jogging suits and tennis hats play out the daily pinochle game.

"Oh, baby, what the white folks are doing to us here--it's a shame," said one retired man in his fifties who "just comes here to play cards."

From around the corner at U and 14th, a man with a brown toothy grin comes to watch the game. Nearby, drug addicts congregate across U street and wait for a connection.

"I've seen the junkies come and the junkies go, and come back again," said Underwood, referring to results of the periodic police sweeps of the area in his 12 years working on the strip. "They just keep pushing them up 14th Street."

And as the addicts are moved farther up 14th Street, new area residents, who once avoided the strip and traveled along 13th or 15th streets, nightly pass the prostitutes and street people without apprehension.

The change is partly a result of the increasing police sweeps, which have made the strip a little safer for customers of businesses that have recently moved to 14th Street and to patrons of The Source Theater and a new theater, The Warehouse Rep.

"It's becoming a hot place," said Mowbray Associates' broker Betty C. Pair, a Logan Circle resident since 1977.

Residential prices range from $75,000 to $100,000 for shells, and $125,000 to $250,000 for already renovated houses.

"A house that sells for $150,000 here would get $250,000 in Dupont Circle," said Stephen Stein, a real estate broker.

Stein said rent for commercial space on 14th Street is about $100 per square foot, compared to $500 a square foot downtown.

Such bargain commercial rates caused Mary Sillers, owner of Hawkeye Art Services, to move her business from Dupont Circle to the 1700 block of 14th Street 18 months ago.

Sillers, who now pays half the rent she paid at the old location for three times the space, admits she takes a lot of ribbing about being on 14th Street.

"It's particularly bad when you say you're framing on 14th Street," she said, laughing.

Sillers' photographer, Evan Sheppard, said they have had no trouble from the neighborhood street people.

"You pass by for awhile and soon the wino on the corner says hello and asks about the dog," said Sheppard. "It's like we've been accepted as part of the neighborhood."

"They'll even watch the store for me," Sillers said. "I'll come back from an errand and they will say, 'Hey, two people came looking for you, but we told them you'd be right back.' "

Sillers, who lived in her store for the first year, believes the area is safer than Georgetown.

"People are actually more aware here, so nothing is really going to happen," she said, adding that the week she moved into the 14th Street storefront she was robbed on a trip to Georgetown.

Closer to Logan Circle is The Canal Company, an antique shop that opened a branch at 1612 14th St. NW two years ago. Owner Jeff Yudin remembers initial area resentment, especially when he refused to buy stolen property.

"They would yell, 'What in the hell are you doing here if you're not going to take hot stuff? Why don't you stay in your own part of town?' "

But Yudin said it was no different from customers in Georgetown who admired his antiques but refused to buy because they did not like the Foundry Building.

Sillers and Yudin agreed that while walk-in clients are not as numerous as in Georgetown, a firm's clientele will follow it to a new location.

The success of The Source Theatre Company gives credence to that theory. In its two years at 1809 14th St. NW, the company not only has brought in patrons from more affluent parts of the city and the suburbs but has founded a second group, The Warehouse Rep, a few doors closer to U street at 1825 14th St. NW.

"Bart Whiteman, our artistic director, had a vision. He wanted to see this place change and be like any other busy street in the city," said general manager Barbara Jones.

Jones said the Source chose 14th Street for the same reasons other companies had: huge space, low rent and a central location.

Despite this, old-timers such as Paul Mattos, whose automotive paint store at 1811 14th St NW was started 20 years ago, think the people next door at Source are crazy.

"They're really nice people, but I just don't follow their logic," said Mattos, who remembers when 14th Street was a center for big auto showrooms and supply firms. "To bring theater to an area like this--I just can't fathom it."

Several area residents pointed to the transformation of a burned-out building at 14th and Rhode Island Avenue into a 7-Eleven store, due to open in August, as another sign of progress.

Walter Eutize, real estate manager for Southland Corp., which owns the 7-Eleven chain, said the company decided to open a store there after police assured the company that the area's crime problems were coming under control. He said the general upgrading of the neighborhood was another important factor in the decision to open the franchise.

Not everyone welcomed the prospect of a 7-Eleven store. Underwood, branch manager of Mattos Automotive Paint Store, said he feared the store would become a hangout for teen-agers and addicts.

"Wherever there are junkies they will usually congregate near a store where there are sweets," said Underwood, who remembers the early 1970s, when Mattos' customers were afraid to get out of their cars because of addicts.

Eutize said 7-Eleven does not approve of loiterers or anything else that would endanger its franchises and the company's quarter-million dollar investment.

"It's to our best interest to see the neighborhood improve," he said.

Although several longtime businessmen and others welcomed the transition 14th Street has been making recently, there were some sobering thoughts.

Real estate broker Betty Pair said newcomers such as Hawkeye Art Services and The Source Theater that were willing to pioneer the strip when few other businesses dared to locate there eventually could be forced out by high prices as the area improves--just like the early residents of Logan Circle.

Jones, of The Source Theater, said the theater was looking for a sponsor to buy its building, and Sheppard, a photographer at Hawkeye, is eyeing the company's next move.

"I know it's going to be Eighth and H NE," Sheppard said, referring to another riot-scarred area that has seen little redevelopment.