No one should accuse the Reagan administration of consistency for consistency's sake. After months of saying a loud "No!" to European demands to do something about the overvalued dollar, the administration, in the person of Treasury Secretary James A. Baker III, this week sent out a call for a new international monetary conference.

On March 25, at a breakfast with reporters, President Reagan had firmly rejected the idea.

"This (talk of monetary reform) has come up before in international meetings," the president said. "There are others that look back at Bretton Woods and . . . wonder should we take another look and see (if) there have been distortions, or whether something better an be worked out."

Yet, in a surprise move 18 days later, Baker told the 24-nation OECD here that the United States is willing to consider hosting a high- level conference "to strengthen the current system," if convinced it would be of value.

To be sure, the idea is a bit vague and is put forward only to patch, fix or improve the present system, not to overthrow it for something brand-new. Something more radical is proposed by advocates of a full-scale, Bretton Woods-style conference, like the first one in 1944 that not only created a monetary system from scratch, but launched the still-existing International Monetary Fund and World Bank. Baker's vista is much narrower.

Nonetheless, the administration has made a basic and important change in policy. It did so deliberately, as one high official said privately, "to preempt the French."

For the past two years, French President Francois Mitterrand has desperately tried to get the world to go back to the fixed exchange rates that were set at Bretton Woods. These began to break down in the late 1960s. First, the Vietnam war inflation and then the initial OPEC oil shock in 1973 gave the coup de gr.ace to the Bretton Woods system.

Mitterrand has the quaint idea that Humpty Dumpty can be pasted back together again, and that in a restored world of fixed rates, the franc somehow will be stronger in terms of the dollar. No one else seems to believe that, but the spectacular surge of the dollar in recent months against all currencies has given many others -- not just the French -- a case of the jitters. What happens if the dollar should plunge as quickly as it went up?

After Mitterrand proposed a Bretton Woods II in 1983, the summit countries chartered a study of the monetary system by the finance ministers of the 11 leading nations. With this report about finished (it will be ratified in June) and sensing sober concerns about instability of the monetary system, Baker -- after consulting White House Chief of Staff Donald T. Regan -- decided it was time to move.

"We have no embarrassment in making a shift," a high source said. "In effect, we said, 'Okay, guys, if action is needed in this field, we're going to lead it, not you: The leading economic power is also the leading monetary power.'

In his regime as Treasury boss, Donald Regan had twice come to the brink of a similar proposal. First, more than three years ago, he made headlines by talking of a new monetary conference. Nothing came of that.

Then, after some of the same issues were raised at the London economic summit last year, Regan told reporters that he could see the wisdom in a get-together of major powers to discuss various improvements in the system. He made a formal proposal to that effect at the IMF-World Bank annual meeting in Washington last fall. And this, in fact, was supposed to take place in Washington this week at regular meetings of the IMF and World Bank policy boards. But as a skeptical IMF source said after Baker's initiative: "Everybody's forgotten about Regan's proposal."

There are many ways in which the IMF and the Bank can be strengthened, so as to become better managers of the world's monetary and debt systems -- provided the major nations allow these international bodies to share their political power. It means a more attentive and sympathetic ear to Third World financial and economic needs.

This may be too much to ask of mortal national politicians. But it's worth a try. I'm not sure that this is precisely what Reagan, Baker and Regan have in mind.

But a process has been started, and it could evolve into something constructive. Looking at it just from an American perspective, when the rate of the dollar precipitates a Depression-like crisis in the farm economy, and makes competitive mincemeat out of highly efficient modern industries, something is wrong.

Doubters should be able to tell quickly whether Baker's initiative was merely a ploy to sidetrack the French, or whether the administration means business this time. Perhaps complaints about the dollar from well- placed Republican businessmen-contributors are beginning to pay off.