Step right up! Watch quick-change artists turn the Washington Convention Center into an inaugural ballroom, a showroom for 500 cars, a cafeteria for 2,000 people, a boxing arena, a basketball court!

Slipping out of the garb of one convention into the trappings of another has become almost routine for the 2-year-old convention center, 900 Ninth St. NW, which is fast becoming well-known among organizations across the nation as a meeting place. Last year, more than a million people attended events there. All of this means a harried schedule for workers responsible for setting up and breaking down shows.

Magic reigns behind the bland tan exterior of the two-block-long center. The decor may be an exotic flower garden one day and a showcase for boats the next.

A lot of the activity occurs at night, when the surrounding downtown is quiet. It is then that a small crew of workers may pick up a million of sheets of paper, stack thousands of chairs and pull out dozens of room-creating partitions, only to erect dozens of new partitions to make new rooms for the next client.

The National Education Association (NEA) opened its general assembly at the convention center Friday in the hall where President Reagan danced with his wife through candlelit darkness on Inauguration Day. A room where famed trumpeter Miles Davis performed a couple of weeks ago has been divided in half, with one end becoming a comfortable press office with wooden desks, typewriters, and telephones and the other half a meeting room.

For 24 straight hours before the NEA convention started, the convention center staff worked to set the stage. By the time the conference opened, the workers had set up 10,000 chairs in the assembly hall, arranged 30 meeting rooms and put together a portable cafeteria for 2,000.

"Before they came, we had a . . . Luther Vandross concert," said Elizabeth A. Overstreet, convention center manager of event services. "We tore out everything [12,000 chairs and a stage] so NEA could bring its equipment in.

"It was an all-night operation," said Overstreet. "After the equipment came in we had to put the chairs back down."

Because NEA booked the entire building, the convention center staff also had to tear down an "antique show, a word-processing show and the setup for a boxing match," all of which had taken place 24 hours earlier, said Overstreet.

After each show, rugs and furniture are cleaned by gasoline-powered vacuums. Trash is picked up, bathrooms get a thorough cleaning and toilet paper is restocked. The convention center staff, including management, housekeeping, security, sales and the events unit, totals 140 people.

The real quick-change artists, the events setup crew, are eight full-time workers, all of whom were too busy lately to be interviewed.

"It is the responsibility of the convention center to turn over to clients the rental parts of the building, clean and ready for them to move into," said Alan Grip, assistant general manager. "We are also responsible for supplying and setting up tables and chairs."

Sometimes the size of the job is mind-boggling. For the National Radiological Society show last year, 320 phone lines were installed, 9,000 chairs were set up and 200 pitchers of water a day were served. Meeting rooms were changed, broken down and set up again 45 times during a two-week period.

The convention center has its own telephone system, which can provide up to 2,400 lines, Grip said. "We provide phones and staff to install them and we fulfill all power requirements, installing utility lines."

Clients supply their own decorations, including booths, information desks, exhibits and some partitions. They are responsible for trucking in their own equipment and setting it up, Grip said.

"It took six days for NEA to move in; their show lasts six days, and it will take two days for them to move out," he said. On one move-in day last week a cherry-picker crane raised a worker to put finishing touches on a red, white and blue stage backdrop while trucks and forklifts grunted under the weight of exhibit materials on the center's back dock.

Planning for the NEA conference started a year ago, with Overstreet working closely with NEA staff. "It's a phase kind of activity," she said of the planning. "It got real intense . . . as details fell into place on their side. It's a matter of taking a client's needs and fitting them into what will work in our building."

"After two years of conferences and shows, I've never seen a hall look the same way twice," said Grip.

Last year, the largest indoor sit-down dinner in the history of the nation was held at the convention center, an Alpha Kappa Alpha sorority dinner for more than 7,000. The convention center's in-house caterers kept two refrigerated tractor-trailer trucks nearby to cool 1,203 pounds of lettuce and 15,400 cherry tomatoes for the salad.

For the NEA conference, the caterers set up a portable cafeteria that seats 2,000, said Bibiche Knaus, administrative assistant to the manager of Convention Center Caterers.

"It took three days to prepare everything," she said. "We set up tables, draperies, plants and gazebos. We'll have eight serving lines and 45 people working, including six cashiers . . . .

"This is in addition to our regular cafeteria which seats about 500 and another mini-cafeteria," said Knaus. "We have two concession stands serving sodas, hot dogs, pretzels and prewrapped sandwiches; our lounge, which serves a different lunch each day, and portable concessions for early birds offering coffee and danish from around 6:30 a.m. to 11 a.m."

The center's principal market has been national conventions and trade shows, said Grip, offering these statistics: In 1983, its first year of operation, the convention center attracted 981,500 people. Last year the attendance was more than 1.1 million. For the first quarter of this year attendance was 556,000. Also, last year visitors to the convention center spent $125 million in the District.

There are some shows the convention center can't handle. There will never be a circus there. "We're not set up to do that kind of show," said Grip, frowning. "You can never get rid of that animal smell."

Overstreet had to refuse a recent client's request to recreate the opening of the Olympic Games because, as she explained, "Open torches aren't allowed."