THE AMERICAN change on Cambodia comes not a moment too soon to offer a saving humanitarian bonus to desperate boat people from Vietnam -- and to soften an acute American policy dilemma. Arriving in Hong Kong, Malaysia and elsewhere in Southeast Asia, thousands of boat people have been ''screened out'' as economic migrants rather than political refugees, denied entry by the countries that resettle Vietnamese refugees, and as a result faced with being sent back to Vietnam against their will. Through the spring and summer, the ''first-asylum'' countries demanded that the United States, which has opposed involuntary repatriation, either relax its objections or otherwise arrange to take screened-out migrants off their hands. Only now can this difficult choice perhaps be eased.

All along, the issue of the boat people has rested in the troubled relationship between the United States and Vietnam. Most of the early boat people, after the American defeat in 1975, had had an association with the United States which left them tainted and despised in the victorious Communist regime's eyes. Most of the more recent boat people have been fleeing not simply the rigid political controls of Vietnam but also the harsh economic conditions arising in part from the American-enforced embargo. With Washington's decision to open negotiations with Hanoi on a Cambodia settlement, however, comes the chance for an overall thaw. And while all this lies in an uncertain future, a Vietnam that is reviving economically and perhaps also relaxing politically is not going to generate refugees or migrants in the old ways.

The most immediate aspect of the crush on the boat people has been the human predicament of Vietnamese who had taken the awesome risks of flight, only to be threatened with involuntary return. No less ominous over the long run, however, is the erosion of the basic principle that nothing should stand in the way of relief for genuine refugees. For good reason, the United States objected to forcible repatriation -- even of economic migrants -- to a Communist police state. One unwelcome result, however, was a growing readiness by first-asylum countries to reject or ''push off'' all boat people, political refugees as well as economic migrants, lest by letting any of them land they incur a responsibility to let all of them stay. This is the specter that stands to be banished by the new American opening to Vietnam.