BOSTON — AS the Peace Corps passes its mantle to a new director, it faces daunting challenges and historic opportunities."The Peace Corps could be the human equivalent of the Marshall Plan, allowing Eastern European countries to benefit from the skills Americans have," says John Coin, associate director of the agency from 1962-67. Paul Coverdell, the affable Georgian who led the agency during a time of unprecedented change in Eastern Europe and the Soviet Union, resigned yesterday. He departs three days after the Ukraine became the first Soviet republic to officially request Peace Corps volunteers. This decade is already proving to be demanding for the corps. Since May 1989, when Mr. Coverdell became director, the agency has entered or returned to almost 30 new countries, including seven in Africa, six in Latin America, and six in Central Europe. The agency now has 7,000 volunteers in about 90 countries. If the current trend continues, the corps could be serving more than 100 countries with 10,000 volunteers by 1995. Also, Congress has approved budget increases for the agency each year since 1989. The projected Peace Corps budget for 1992 is $200 million, a $14 million increase over 1991. After two decades of small budgets and little interest from the White House and Congress, this expansion suggests renewed interest from Washington in the corps. It also prompts some questions: What kind of country should Peace Corps serve and who should lead the agency? Some Coverdell critics say that programs in Central Europe come at the expense of programs in lesser developed countries. "It's robbing Peter to pay Paul," says Kitty Thuermer of Appropriate Technology, a development firm. "Money needed in Africa is siphoned off for more glamorous and politically expedient countries." Coverdell disagrees. The Soviet-bloc countries were "the first countries sought by [founders] President Kennedy and Sargent Shriver," he says. "But that couldn't happen. Now, the dream is being fulfilled." The outgoing director also notes that, from 1989-91, the corps's budget for its Africa programs has increased by more than $8 million and Latin America by almost $5 million. In addition to the Ukraine's invitation announced last Friday, the three Baltic republics also have expressed interest in hosting Peace Corps volunteers. That, combined with the corps's recent entry into Poland, Hungary, Czechoslovakia, Romania, and Bulgaria, are just the beginning. "There have been discussions between the Peace Corps, the State Department, and the White House" about sending 500 volunteers to Russia, says Jon Keeton, director of international research and development at Peace Corps. He says such a program would cost about $7.5 million. "Volunteers embody the very democratic precepts that are essential for change in these countries," Mr. Keeton says. The agency could provide programs in English, environmental management, microenterprise, and agriculture, he says. If critics are tentative about the direction of Peace Corps's expansion, they are certain about the qualifications needed for its leadership. "The director has to be a returned volunteer," says Mr. Coin. "We've produced some very qualified individuals, but none have ever been tapped for the directorship." "The director must have international and government experience," says Lyn Gray, executive director of the National Board of Returned Peace Corps Volunteers. "The nation would be well served to have a nomination with Peace Corps experience." Coverdell has been criticized for his lack of Peace Corps experience prior to serving. President Bush has nominated Elaine Chao, United States deputy director of transportation, to succeed Coverdell, who is leaving to consider running for the US Senate from Georgia. Ms. Chao's Senate confirmation hearings will likely begin later this week.