AS THE PROSPECTS for world trade and finance have become visibly more dangerous, one good and hopeful change is taking place. Governments -- and particularly the United States government -- have begun to come to terms with the dimensions of these threats with more speed and more realism than seemed possible a couple of months ago. All of them, but most notably the Reagan administration, are tuning down the accusatory bombast and talking pragmatically about cooperation. That in itself is a major contribution to stability.

By last weekend, the secretary of agriculture, John Block, was deprecating the talk about trade wars in farm exports. After long conversations in Brussels, Mr. Block has evidently arrived at a sharper appreciation of the costs involved in losing the national temper. Instead, there is to be an intensive study of the European export subsidies, promptly followed by negotiations.

The secretary of the Treasury, Donald T. Regan, has been converted to the view that the world's credit markets are not necessarily going to right themselves automatically, and the present safeguards are perilously inadequate. His trip to Brazil earlier this month with President Reagan must have been a highly illuminating experience. He now supports a large increase in the lending resources of the International Monetary Fund -- a wise precaution that, three months ago, the Treasury was fighting vigorously.

The shift in the focus of the international debt crisis from Poland to Latin America makes it much easier for the Reagan administration to respond usefully. In Mexico and Brazil, the United States is dealing with countries that Mr. Reagan considers to be good friends, entitled to help in their present travail. The financial issues are no longer entangled with ideological and strategic preoccupations with the Soviets.

What needs to be done now? Expanding the International Monetary Fund's lending authority clearly comes first. Some of the banks are also organizing a better system of information on debts. It has been dismaying, over the past year, repeatedly to discover that the first official estimates of a country's debts turned out to be far too low.

Beyond that, it may be necessary to convert some of the Latin countries' mountain of short-term bank debt into long-term debt. It would have to be done by one of the international institutions; the banks need it badly enough that they would accept a substantial reduction in interest rates to get it. That suggests the possibility of a deal benefiting both lenders and borrowers. But certainly anything along these unorthodox lines would require the most active and forceful participation of the Reagan administration. Paradoxically, because the possibilities of trade wars and financial collapses have seemed much more menacing in recent weeks, the United States government has swung to a much more sensible and constructive position--and, in that very significant respect, things are already improving.