The latest switch, which took effect last week, has been made to provide maternity cover for head of national security comms Emily Hands for a year. Bower will cover Hands' Afghanistan and NATO summit brief.
Marchbank retains his role as head of news for the Prime Minister's office while Bower retains her role as the PM's principal foreign affairs and defence press adviser.
The deputy spokesman conducts one of the nine weekly lobby briefings, with the PM's spokesman Jean-Christophe Gray doing eight.
Bower is the fourth person to occupy the role since the start of 2013, when Vickie Sheriff landed the job of director of comms at the Department for Transport.
Sheriff was replaced on a temporary basis by Christian Cubitt in March, before Marchbank was handed the role on a permanent basis in August on loan from head of news at the Ministry of Justice.
The Prime Minister’s spokesman Jean-Christophe Gray said: "These changes will ensure that we can successfully manage Emily’s wide-ranging portfolio over the busy and important months ahead. Scot has a strong background in security matters and Helen’s expanded role will make full use of her extensive knowledge of foreign affairs, and experience of working with the Westminster Lobby on foreign and defence matters."
Bower secured the job of chief press officer for foreign affairs and defence in May 2011, moving from head of the press office at the Foreign and Commonwealth Office's UK representation to the EU, based in Brussels.