The Carter administration is shaking up its top-level Latin American diplomatic ranks, appointing new ambassadors in eight countries. This raft of appointments brings to 18 the ambassadorial changes made in Latin America during the last year and a half.

The reasons for the shakeup differ from country to country, but most agree the sheer number of changes is unusual. Some say that William G. Bowdier, who has headed the State Department's Latin American division since January, is building his own diplomatic team.

Other officials maintain that events have forced the administration's hand. They point to violence and political turmoil in Central America, where the administration is trying to implement a policy recognizing the need for change. aThere are also some individual circumstances, as in Colombia, where the administration needs to replace former ambassador Diego Asencio, who was held hostage for 61 days this spring.

The net result of the changes, coming just four months before the presidential election, will be fewer political appointees in ambassadorial positions and more career diplomats.

Most of the appointments are still unofficial. All require Senate confirmation.

The prospective changes include:

Guatemala. Frank Ortiz is being supplanted by the U.S. ambassador to Chile, George Landau, because the top brass at the State Department feel Landau is more capable of carrying out U.S. policy in tumultuous Guatemala. l

Chile. Landau will be replaced by John Bushnell, a deputy assistant secretary under Bowdler. Bushnell's reputation rests on his economic and trade policy expertise, his bureaucratic lasting power and his extensive diplomatic experience.

His appointment is already stirring concern among human rights activists, who want the administration to send a negative signal to the repressive Augusto Pinochet regime by delaying the Chile appointment for a while. They also see Bushnell as hostile to their human rights campaign.

Argentina. Harry Schlaudeman, now ambassador to Peru, will replace Raul Castro, who has resigned his post to join the Carter presidential campaign.

The appointment of Schlaudeman, an experienced diplomat who served as deputy assistant secretary of state under President Ford, may be aimed at improving U.S.-Argentina relations. Concerned by growing Soviet ties with Argentina, the administration hopes to persuade Congress to relax its prohibition on giving military assistance to Argentina, a major human rights violator.

Schlaudeman, who has a reputation for following policy orders with alacrity, may be in place to carry out the administration's policy shift if it comes about.

As with Bushnell, human rights activists question Schlaudeman's commitment to human rights issues.

Peru. Schlaudeman may be replaced by Samuel Eaton, a deputy assistant secretary of state.

Uruguay. Robert S. Gershenson, a deputy assistant secretary of state for personnel, will replace Lyle Lane.

A number of Foreign Service officers are critical of Gershenson's nomination. Gershenson, who has been closely involved in the administration's legislative overhaul of the Foreign Service, has alienated members of the service who favor a stronger provision for a grievance procedure. They are also critical of how he has handled his job as the person with major responsibilities for administering promotions and assignments at the State Department.

Paraguay. Lane is being transferred to Paraguay after spending a year in Uruguay.

Honduras. Jack R. Binns, now in Costa Rica, will replace political appointee Mari-Luci Jaramillo, who is returning to Washington as a deputy assistant secretary of state.

Colombia. Diego Asencio, who will remain in Washington as assistant secretary for consular affairs, may be replaced by Edwin Corr, now a deputy assistant secretary of state for the Bureau of Narcotics Matters.