The National Old-Timers Baseball Classic that has been played at RFK Stadium since its inception six years ago, will be moved to Buffalo, N.Y., next year because of construction of 2,000 additional seats at RFK Stadium, Dick Cecil, the game's executive director, said yesterday.

Calling the move "a friendly thing" with the D.C. Armory Board, Cecil said the move was necessary "because we were told major construction likely would take place in the summer . . . Washington's been a good city for us."

But Cecil declined to say positively the game would return to Washington. "We're dealing one year at a time," he said. "We always do. We were told there would be construction at RFK, so we made other plans."

The game will be played June 20 as the dedication game for Buffalo's new 20,000-seat Pilot Stadium, which can be expanded to 40,000 if that city gets a major league franchise.

In Washington, Bob Downey, top aide to Starplex general manager Jim Dalrymple, said construction would start in late winter or early spring and would take eight to nine months to complete. As a result, the job may require two football off-seasons to finish, Downey said.

In addition to the seats being added to the stadium, another benefit will be the ability to convert the stadium from football to baseball more quickly. Currently, that is impossible because of a very short left-field fence.

For an exhibition game last spring, a wooden wall was constructed in left field to make a more imposing home-run target.