Nixon is moving to the position of executive deputy chairman, from 25 February next year, where he will focus on "strategic innovation and development".
He said he had chosen a new role that will allow him to continue to use his entrepreneurial skills.
Plumb will be appointed to the board of Moneysupermarket.com as an executive director from 1 January.
The group has also strengthened its board with two new directors. Managing director of Travelsupermarket.com Graham Donoghue will become executive director of Moneysupermarket.com and easyJet regional general manager UK, David Osbourne, will become marketing director.
Plumb, Donoghue and Osbourne, together with executive director Paul Doughty and group HR director Alan Cairns, will now comprise the senior executive team.
Gerald Corbett, chairman of the board of Moneysupermarket.com, said Plumb is a "tried and tested general manager" in international, marketing-intensive businesses.
In a trading update, the board said that although trading in the fourth quarter had continued to weaken, the group's trading position remains in line with the interim management statement of 13 November and added that it is confident of being "in the range of analysts' expectations for 2008".
A further update will be provided with the announcement of the 2008 results. The board said the company's financial position is strong with no debt and cash balances of more than £70m.
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