THIS IS A tale of two paintings, both by Stuart Davis. The Corcoran owned one of them for half a dozen years. It is no longer there. It's been traded for the other.

The switch has cost the gallery $50,000. Because art museum curators shy away from de-accessioning, it required courage, too.

Davis number one, called "Cafe', Place des Vosges," is a relatively lighthearted souvenir of Paris. The Halpert Foundation gave it to the Corcoran in memory of Edith Gregor Halpert, the painter's longtime dealer, in 1975.

Davis number two, called "Study for Swing Landscape," is jazzier and brighter and considerably more abstract. It was made in 1938 while Davis, then employed by the Federal Art Project of the WPA, was working out the scheme for the left half of "Swing Landscape," a large and jaunty mural that is now on view at Indiana University. The flat and rhythmic forms of the Corcoran's new picture hint at masts and chains and pilings and other harbor elements. Davis was as fond of ships as he was of jazz, and the word "swing" in his title simultaneously suggests both '30s big-band music and what boats do at anchor. Davis did like puns.

Both these Davises are worth a lot of money. While hanging at the Corcoran, the cafe' scene was valued at $200,000. New York dealer Barbara Mathes, who owned the mural study -- and had a buyer for the Paris scene--argued that her painting was worth even more. Negotiations for the trade required many months.

"My interest in abstractions is practically zero," Davis used to say. He stubbornly insisted that he was a realist, though most historians nowadays tend to disagree. They like to think of Davis (1894-1964) as a playful and original prophet of the new. "He was one of the key transitional figures who helped move provincial American Modernism toward Abstract Expressionism," says the Corcoran's Jane Livingston. "He was also the quintessential proto-pop American painter. He worked on both frontiers."

"Study for Swing Landscape" may look a little muddy in black-and-white reproduction, but seen in life its colors appear rainbow bright. With its jaunty reds and yellows, and hard-edged panels of flat color, it looks like a prediction of Washington Color Painting. That's one reason it is here.

Another is the splendid way it complements the other early abstract paintings in the Corcoran's collection. It seems a sort of cousin to the equally hard-edged Patrick Henry Bruce (1924) beside which it now hangs.

Dealer Edith Halpert, who died in 1970, once flirted with the Corcoran. Twenty years ago she said she'd give the gallery her O'Keeffes and her Demuths, her Sheelers and her Shahns, and 1,000 other pictures if they'd build a special wing. They didn't have the money, so she sold them instead.

Because tastes and values change, trading up, in time, may look like trading down. Curators know well that it's a risky business. "We would have loved to have both of them," said Livingston. "But we don't have that kind of money. When we compared the pictures, we felt we had no choice." "Study for Swing Landscape" is now on display on the gallery's main floor.