THE NEW PROTEST rallies in South Korea indicate that the concessions made by President Chun Doo Hwan were too little and came too late to end the turmoil, turn political activity back into channels of negotiation and convince the United States that the leadership has matters in hand. The rallies suggest that the students may stay in the streets longer and that, under their goad, the moderate opposition may intensify efforts to bring down the regime. Whether the army will leave the job of enforcing order with the police or even whether the army will stay united behind President Chun and the fellow former general he has named as his successor is up in the air. The outlook is for a long, explosive summer in Seoul.

Korea, then, is launched into great uncertainty. The United States did not expect this development. But upheaval has come, and with it arrives new risk to the American interest in a dependency's welfare and in the peace of the region -- for across a near and fragile line sits the menacing regime of North Korea.

For decades Washington's challenge in dealing with friendly autocratic governments has been to encourage essential change but not summary change: progress toward democracy but not a loss of control leading on to leftist or anti-American regimes. In recent years, the Philippines stands as a provisional success story. Iran and Nicaragua stand as disasters -- Carter-era disasters that Ronald Reagan rode to the White House pledging never to repeat. Korea and to a lesser extent Panama are his hard cases.

For anticipation in Korea, the Reagan administration gets low grades. Its catch-up game is more promising. Though often criticized for instinctively favoring considerations of strategy and security over considerations of democracy and human rights, it is aligning itself pragmatically in Korea with elements of peaceful change.

The administration detaches itself from the Korean military in one dimension by preemptively denouncing the use of force on civilians and any turn to a military coup. It detaches itself in a second dimension by offering public favor for a plan to have President Chun replaced at the end of his term next February not by his handpicked comrade but by an interim figure chosen in broad consultations. So far its pressures have been in the right direction. The question is whether it can bring the Chun government along with it without helping to incite military revolt, worse repression and the predictable reaction to it.