Former Tory chancellor Lord Lawson has been named as the chairman of a group campaigning for the UK to leave the EU.
The 83-year-old will chair Vote Leave, one of two groups vying to be the official Out campaign in the EU referendum.
The group's CEO Matthew Elliott, campaign director Dominic Cummings and company secretary Victoria Woodcock are leaving the board.
These changes were previously planned, Vote Leave said.
It said the three members of the executive team would still "attend and contribute to board meetings" and that Mr Elliott and Mr Cummings were still in charge of the campaign.
Some Conservative MPs have expressed reservations about Mr Cummings - a former adviser to Michael Gove, when he was education secretary, who strongly divides opinion amongst Conservatives - BBC political correspondent Chris Mason said.
Asked about the reshuffle, a Vote Leave source said: "It's about making sure we move forward as happy ship with the capacity to expand."
Lord Lawson is also president of the Conservatives for Britain group.
"I am confident that other senior figures will step forward to support the Vote Leave campaign, but I am happy to help the board and campaign team make the appropriate decisions in the crucial weeks ahead," said Lord Lawson.
He replaces Labour donor and home shopping magnate John Mills as Vote Leave's chairman. Mr Mills will become deputy chairman, focusing on the work of the Labour Leave group in targeting centre-left voters.
Vote Leave is competing with Leave.EU to be officially designated as the Out campaign for the UK's in-out referendum.
Leave.EU said the departures of Mr Cummings and Mr Elliott from the board means "major obstacles" to a merger between the groups have been removed.
But speaking on BBC Newsnight, Tory MP Steve Baker, whose Conservatives for Britain group is affiliated to Vote Leave, said he did not think this was possible, because of a "genuine disagreement about strategy and tactics".
"I believe Vote Leave will win the designation, I believe Vote Leave will win the referendum," he added.
Britain Stronger In Europe seized on the shake-up in Vote Leave's management structure to claim the campaign had "descended into all-out farce" and that the "demotion" of John Mills was "the final nail in the coffin for their claims to represent a truly cross-party coalition".