The Reagan White House enters its second year with a changed cast of characters and signs that initial euphoria over the troika heading the staff has given way to a cordial but sometimes tense working relationship.

The replacement of Richard V. Allen with William P. Clark as national security adviser is the most visible and may well become the most important change. In addition to removing an issue that had divided Reagan's "Big Three" for weeks, the shift will have a major impact on the relationships between the president's top advisers.

Whether Clark performs well or not, his presence changes the nature of the top Reagan staff. Before, only three reported directly to Reagan: counselor Edwin Meese III, chief of staff James A. Baker III and deputy chief of staff Michael K. Deaver. Now there are four.

Presidential counselor Meese has a smaller writ, because he no longer will be the funnel through which all national security reporting goes to the president. Baker has expanded his role, most notably in planning legislative strategy, and emerged from the first year stronger than he began it.

Of the men who moved into the White House with Reagan just short of a year ago, Allen and congressional liaison chief Max L. Friedersdorf are gone. Political director Lyn Nofziger is leaving this month. Deaver has announced he will leave in a year.

A number of outside observers had predicted early in the administration that a three-way split of power could not continue amicably. For the most part, they were proved wrong last year.

But as Clark arrives on board, well-informed sources report increasing tensions, particularly between Meese and Baker.

Some White House officials look to Clark's genial presence to improve the way things work. "Clark gives this place a lot of weight at a time it really needs it," said one Clark admirer.

The handling of the Allen affair brought about the most public disagreement between Meese (who initially wanted to keep Allen) and Baker (who decided early that Allen should go), but they have come down on different sides of other issues. In August, Baker pushed for significant cuts in defense spending. Meese did not and never called the president's attention to the arguments for reductions until Reagan had made up his mind against them.

Meese and Deaver came out of long years of California and campaign service with Reagan. Baker worked for former president Ford and ran Vice President Bush's presidential campaign in 1980.

Baker and Deaver have moved closer together, however, during their first year in the White House, according to White House officials.

Shifts in the relationships of the Big Three (Meese, Baker and Deaver) have been highlighted by the way junior staff members have developed their roles. The two theoretically most powerful Meese deputies--Allen and domestic adviser Martin Anderson--have not become dominant figures in the Reagan White House.

Questions were raised about Allen's performance long before the revelations of his dealings with Japanese friends and reporters that ultimately sped his departure.

Anderson, head of the Office of Policy Development, has kept a low profile and reportedly has been bypassed on some issues on which he would normally play a leading role. What one official described as a "back channel" has been established through presidential assistants Richard Darman, who reports to Baker, and Craig Fuller, who reports to Meese.

Darman and Fuller have become increasingly visible and powerful over the past year. One White House official put it drily: "They have gotten more and more into policy."

The result of all the shifts, particularly the departure of Nofziger, is to give the Reagan White House a distinctly non-conservative coloring. "Nofziger was a critical asset," one official said. He has been through so many political wars with Reagan that "he knows who has been loyal over the years," the official said.

Nofziger also had the trust of the Republican conservatives and without him the Reagan White House is more vulnerable to their accusations that the president is abandoning the ideology and the people who got him to the White House. No one else in the White House has such good ties to the right.

It is more difficult to calculate the likely effect of Deaver's statement that he intends to resign at the end of the year. His extraordinarily close relationship with the Reagans and the non-policy-making nature of his job make it unlikely that he will be seen as weaker because he is a lame duck.

"So much depends on how Clark works out," one White House official said. If Clark is a success, he added, it will lighten the workload of all the Big Three.

If Clark can succeed in avoiding the kind of clashes Allen frequently had with Secretary of State Alexander M. Haig Jr. it would open the way for the White House to become more involved in the explication of foreign policy, according to a White House official who would like to see that happen.

He is optimistic. "I've watched Haig and Clark in meetings. They are supportive of each other. I don't anticipate the kind of sparks we had before."

For better or worse, the staff is set for the foreseeable future. "There won't be any more changes at the top level," an official said.