Recent turnover on Metro’s board of directors could undermine the panel’s effectiveness as it tries to overhaul its governing rules and guide the $2.5 billion transit authority beyond the troubles of the past, some members and watchdog groups say.

With two new members from the District set to be sworn in Thursday, eight new members will have joined the 16-seat board this year. Two seats remain vacant.

Board members also are waiting to see which of the four Virginia representatives will have to step aside to make way for an appointee to be named by Gov. Robert F. McDonnell.

Last week, D.C. Council Chairman Kwame R. Brown shuffled council committee assignments and removed member Tommy Wells (Ward 6) as one of the District’s voting representatives on the Metro board. Wells joined the board in January.

Jeff McKay, an alternate director on Metro’s board and a Fairfax County supervisor, said the recent changes “create cronyism and an allegiance to one person instead of the 1 million riders a day we’re supposed to be serving.”

“Instability is never a good contributor in getting things done,” he said. “We have to stop meeting and greeting new board members and move on with fixing the agency. Every time there’s a bump in the road there’s a minor setback for the progress we’re making.”

However, Metro board member William Euille, who is also mayor of Alexandria, said the changes will not impede the board’s work.

“The train will not be slowed down at all,” he wrote in an e-mail. “There is too much work to be done.”

At Thursday’s meeting, council member Muriel Bowser (D-Ward 4) and lobbyist Thomas J. Bulger will replace Wells and council member Michael A. Brown (D-At Large), respectively. Brown announced last month that he would leave the board.

“We’d just calmed everything down and we just did a budget without any drama,” said Wells. “The changes diminish and undercut the seriousness of D.C. appointees. . . . It politicizes a board that really shouldn’t be.”

Bulger said the Metro board has undergone “significant” turnover — change that he considers “a good thing. It brings fresh ideas and more energy.”

Another seat could become vacant if Michael Barnes, a voting principal director from Maryland who joined the board in April, resigns. Barnes recently said that he would be willing to step down, a response to a debate about whether the board is too large.

Maryland, the District, Virginia and the federal government each appoint two voting principal directors and two non-voting alternates to the Metro board. Fourteen seats are filled. An alternate seat for the federal government and one for the District remain vacant.

The board has set out to make new rules for how it governs following local and federal reports critical of past practices, including a recent assessment from the Government Accountability Office. Members have established bylaws, wrestled with how big the board should be and discussed whether jurisdictions should have common standards for appointing and compensating members.

Jim Dinegar, chief executive of the Greater Washington Board of Trade, which last year released a joint report with the Metropolitan Washington Council of Governments that criticized Metro’s structure as outdated, said the board has made progress. He cautioned against a trend of high turnover.

“I’d want to see them settle in and plan out the remainder of the year,” he said. “We’ve never asked Metro to be a mini Congress where it is a certain number of seats for every state.” His group has pushed for shrinking the size of the board to 12.

The two new board members from the District said they are eager to help improve Metro.

Bulger said he wants to assist as the agency implements safety recommendations made by the National Transportation Safety Board after the 2009 Red Line crash and help decide how Metro can grow with the region’s population.

Bowser said her top priorities are to help Metro deal with its safety issues and improve the quality of service. She said she also wants to work to make Metrobus “more accessible, more reliable” to riders.

“I want to make sure Metro’s board represents all citizens in making sure we move the system forward,” she said.