U.S. relations with several countries in Eastern Europe appear to be markedly better under the Carter administration than they were before because of a major shift in how Washington handles the Kremlin's allies.

Experienced American and European diplomats throughout the region are awarding high marks, at least tentatively, the White House for what adds up to a potentially dramatic yet subtly carried out policy change that was never really announced.

The change involves two key elements.

A move away from what is viewed by a perhaps "excessive preoccupation" during the Nixon-Kissinger years on dealing almost exclusively with the Soviet Union, to a postion now in which more attention is paid to individual countries within the seven-nation Soviet bloc and to independent communist states, such as Yugoslavia, outside that alliance.

Within that shift lies a second one: the effort to relate to the people and societies in those countries rather than dealing only with the governments.

There is also a third level, one that is more influenced by political conditions in the East than are the other two.

Thus countries such as Hungary, Poland and Romania are being treated differently from the more hard-line and less flexible states such as Czechoslovakis, East Germany and Bulgaria. Hungary is the least repressive country in the Soviet-led Warsaw Pact. Romania has a foreign policy that often strays from the Kremlin line.Poland has the most independent-minded population as well as the interest of 6 million Polish-American voters in the United States.

It is too early to say, many diplomats report, what the United States will get in return for its redirected interest.

"You don't really get something back so fast in response to events over just a single year," cautioned one State Department official. "You've got to use it to deepen and broaden those relations and keep them going in the right direction."

"Maybe the East Europeans are just telling us what they think we want to hear," in return for badly needed Western economic credit and technology, added another observer.

Still, the sense of a positive trend is very widespread.

"Whether is deserves it or not," says an analyst who has watched Eastern Europe for several U.S. administrations," the U.S. has more respect as a symbol these days in Eastern Europe, probably more than it has in Western Europe.

"The East Europeans have always felt themselves either ignored or exploited. But when they are convinced someone shows interest for their own sake, they finds ways of showing it. Maybe it's naivete or wishful thinking, but under this administration the pendulum seems to hae swung back and they do seem to be showing interest in us."

Many diplomats interviewed tended to call the Carter administration policy "remodeled" rather than "new", since some of its ingredients have been around for a long time. Yet there seems to be a general willingness to credit the White House with putting the older prospects into action and creating an atmosphere where new initiatives could be suggested.

Collectively, much has happened since Carter, at a London summit meeting in May, talked about "drawing the nations of Eastern Europe into cooperative undertakings:"

Early next month, Yugoslavia's President Tito will pay a visit to the White House, though Tito traveled to Washington in 1971 and former Presidents Nixon and Ford visited here, this visit comes at a time of visibly improved relations with the United States. It follows a trip here by Defense Secretary Harold Brown in October, the first by a U.S. defense chief, in which was begun a "modest" yet politically important program to resume selling American military equipment to Yugoslavia.

Later this spring, informed sources say, President Nicolae Ceausescu of Romania - a country that receivced substantial U.S. aid last year after an earthquake - will also visit Washington.

Last month, the United States returned to Hungary the sacred crown of Saint Stephen, which is now drawing heavy crowds to a public display in Budapest. This is an example of something that has been thought about for many years but not acted upon until now. Last fall, evangelist preacher Bily Graham also made an unofficial yet unprecedented trip through the Hungarian capital and countryside.

In December, Carter visited Warsaw and held a press conference, the first in a communist bloc country by an American president. It was covered by Polish television and newspapers. Carter also announced extension of additional credits for badly needed feed grains to help ease a lingering and potentially explosive meat shortage in that country.

The idea seems to be to use these various moves, some of which have popular and emotional as well as governmental appeal. To help loosen the binds of these countries to Moscow and give them some maneuvering room without tampering dangerously with the unquestioned Soviet dominance over this area.

Aside from economic needs and a generally improved American image after Vietnam and Watergate cited by many East European officials, there may be other important factors at work.

"I think there clearly are apprehensions in Eastern Europe over the succession in the Soviet Union after Brezhnev goes," said one British analyst.

"Certainly it is haing its effects on everything the East European states do and I feel it is making these countries almost instinctively want to have better relations with the United States and maybe with West Germany and France, too. It's the fear of the unknown and a search for at least some stability in their relationships. They don't know what the hell is going to pop up in the Soviet Union."

Though the other three Warsaw Pact states - Czechoslovakia, East Germany and Bulgaria - are much further from tangibly improved relations with the United States and are the most officially devout of all the Soviet pact allies, there are some small signs of improvement here as well.

The first cultural exchange agreement has been signed with Bulgaria, and East Germany has proposed a series of trade and cultural contacts and has been moving to settle some family reunification cases with the United States.

Czechoslovakia is the most difficult problem, encumbered by a hard-line, repressive government that even East European communists fault as unimaginative and excessively insecure in handling problems with dissidents.

Even here, however, there are hints of movements.

One hears more speculation these days among diplomats that the Carter administration, in a move similar to return of the Hungarian crown, will at some point quietly move to bargain with Prague for return of nearly 20 tons of gold that was taken by U.S. troops from German-occupied Czechoslovakia at the end of World War II and has, like the Hungarian crown, been in Fort Knox ever since.

Last summer, American banks participated for the first time in the floating of a $150 million loan to Prague and this summer, the Czechoslovak president and Communist Party chief, Gustav Husak, will venture for the first time into a Western country - West Germany - since taking power 10 years ago after Soviet tanks ended the liberal experiments of Alexander Dubcek.

American diplomats in Prague are reportedly having somewhat better access to Communist officials, but most diplomats assess Prague's moves toward the United States as "very tentative."

A major problem is rough treatment and banning of many American journalists who have attempted to report the Charter 77 human rights campaign in Czechoslovakia.

Diplomats believe that will not really improve in 1978 since that is the tenth anniversary of the Soviet intervention and Czechoslavak officials seem convinced that most American journalists they might let in will use that theme to describe present conditions there.

Still, some analysts believe Czechoslovak society might also be privately responsive to a warming trend toward the United States.

"While thoughtful people outside the Czech bureaucracy don't want to see the government strengthened, they are probably smart enough to realize that in the situation the country finds itself, it couldn't hurt them any more and it might spin-off some advantages," one said.

Several factors seem to be operating in improved U.S.-Yugoslav relations, as seen by both U.S. aand Belgrade officials.

The Carter administration is assessed here as likely to be more sympathetic toward the so-called nonaligned nations in which Yugoslavia seeks to be considered as a leading force. Washington is also viewed as less inclined to ventures abroad these days.

Yugoslavia, like the rest of Eastern Europe, is increasingly in debt to both the West and the Soviet Union for products and raw materials. The debt to the East, mostly to Moscow, is now about 30 percent of the total. Some Yugoslavs ae uncomfortable with that amount, feeling that it may give Moscow tighter leverage on Belgrade's policies, especially in a post-Tito period.

Yugoslavis broke with the Kremlin in 1948 and has maintained an independent stance ever since, feature that has attracted American interest for 30 years.

The military equipment deal has similar unstated implications because, while reportedly small in value and limited to defensive equipment, it gives the United States at least some links to the Yugoslav military, which is likely to be one of the few stabilizing forces in the wake of Tito's departure.

In a more tangible area, U.S. officials are known to have been pleased by Tito's comment's late in January when, for the first time publicly, he called for Arab states, particularly those surrounding Israel, and the Palestine Liberation Organization, to recognize Israel as a reality in international relations and thus contribute to more trust in the region.