The Arlington County Board's incoming Democratic majority announced yesterday that they will elect the board's senior Democrat, Ellen M. Bozman, as chairman, and John G. Milliken as vice chairman when they take control next month.

The three Democrats, Bozman, Milliken and Mary Margaret Whipple, whose election last month gave the Democrats three of the board's five seats, also said in a joint statement they intend to change the way the Republican majority has run the board.

Chief among the changes, they said, will be attempts to deal with broad, long-range issues, such as health care for the elderly, and a substantive review of the county's 47 advisory commissions and boards.

"We want to help them frame their agendas and make sure they understand where we stand and what our goals are," Milliken said.

"By and large, we've given them no direction in the past," Bozman said.

Bozman, a Democrat-endorsed independent, was reelected to her third term in 1981 and had served as chairman in 1976, when Democrats controlled the board. Milliken, a Democrat elected in 1980, will be appointed to the Northern Virginia Transportation Commission and the Metro board, the Democrats said. Whipple will be serve on the Metropolitan Washington Council of Governments board and take Bozman's seat on the Northern Virginia Planning District Commission, they said.

The three said they would resume the practice of having two regularly scheduled Saturday board meetings each month and said those meetings will begin at 9 a.m., "a more civilized hour for the public" than 8 a.m. when the Republicans began the meetings.

The Democrats also said they want "to reestablish the principle that the membership" of advisory groups "should reflect the makeup of the County Board," a clear indication that they intend to name their supporters to many county boards that are largely run by Republican appointees. Bozman and Milliken have been particularly critical of some of the current Republican majority's appointments to key committees since the election, when GOP Board Chairman Stephen H. Detwiler's loss assured a change in the board's control.

The Republicans "took the cream off the top of the bottle" with their recent appointments to the planning commission and the civil service commission, Milliken said. He added that it would take between 18 months to two years before the makeup of those commissions and dozens of others will reflect the board's new Democratic majority.

Since the election, the Republicans have appointed Robert E. Harrington, Detwiler's campaign manager, to the civil service commission, a body with powers over personnel evaluations, merit raises and reclassifications. They also appointed William G. Buck, a real estate broker and Detwiler campaign contributor, to the planning commission.

Despite their disagreements with the Republicans on these and a handful of other appointments, Bozman and Milliken have voted with the majority on most other recent appointments.