President Reagan has replaced every member of the Joint Chiefs of Staff, giving himself a military advisory body heavy with safe players who would ordinarily be a low-profile, go-along group.

But events are conspiring to force these safe players into high-profile public positions where they may find themselves opposed to the views of the conservative president who appointed them and with whom they feel so comfortable ideologically.

Gen. John W. Vessey Jr., the World War II mud soldier who is now chairman of the Joint Chiefs, already has exhibited the coming dilemmas of the new chiefs. At an open Senate hearing last year, Vessey disclosed that the majority of the chiefs opposed Reagan's "Dense Pack" basing scheme for the MX missile.

But the MX will not go away. Vessey and his fellow chiefs will be put on the spot again soon when congressional committees ask whether they favor Reagan's next idea for basing the MX. A study commission is expected to recommend that the MX be placed in existing Minuteman silos--which the Pentagon already has declared vulnerable to Soviet warheads.

Then there is Reagan's call for a stepped-up effort to develop a foolproof defense against Soviet missiles, with exotic and expensive lasers listed as a leading possibility.

Do the chiefs favor spending billions to do more in this area? If so, what programs will they give up to finance the research? Or do they favor waiting until the weapons they have already ordered under the biggest modernization program since World War II are delivered?

Do they agree with their commander-in-chief on freezing military pay in fiscal 1984? How about the president's hopes of reforming the military retirement system, the sacred cow that is becoming increasingly expensive to feed.

It will be almost impossible in coming months for the chiefs to do the bidding of their individual services, which they command, and still keep lock-step with the president, whom they advise.

Vessey, the only World War II veteran of the chiefs come July 1, when Marine Commandant Robert H. Barrow retires, will be fielding not only these questions but ones on how to make his office more useful and relevant. Critics, including former chairman David C. Jones, have warned that the current Joint Chiefs' structure is a Gulliver tied down by Lilliputians.

Vessey, 60, a Minnesotan who came into the Army as an enlisted man via the National Guard, has Gen. John A. Wickham Jr. as his Army comrade-in-arms on the chiefs. Longtime acquaintances say that Wickham, 54, a West Pointer, has always been the conservative, efficient, detached officer, and predict that he will do little innovating or cage-rattling as Army chief of staff. He will succeed the innovative Gen. E.C. (Shy) Meyer, considered a boy wonder when he was appointed chief four years ago at the age of 50.

To captain the Navy for the next four years, Reagan chose Adm. James D. Watkins, 56, a Naval Academy graduate who is considered "a nuke" within the Navy from his service on nuclear submarines. Colleagues portray him as a deft political operator, but say they doubt that he will make waves for Reagan, partly because he is all but eclipsed by the highly vocal Navy secretary, John F. Lehman Jr.

Former fighter pilot Charles A. Gabriel, 55, is the new Air Force chief of staff, an affable general who got burned by the press through a coincidence of timing. When the subject of freezing military pay was hot, Gabriel was asked by reporters whether he was consulted on Reagan's decision and whether he would have agreed with it if he had been. He answered an honest "no" to both questions. Gabriel is unlikely to poke his head above the cockpit again unless commanded to do so by congressional committees.

Gen. Paul Xavier Kelley, 54, the extrovertish new commandant of the Marine Corps, is a veteran of inner-service warfare from commanding the multi-service Rapid Deployment Force and is well-connected politically with the Washington power structure. He is likely to emerge as the dominant personality on the chiefs, partly for lack of competition.