The election of Dmitri Medvedev as the new President of the Russian Federation was accompanied by confirmation that former president, Vladimir Putin, would serve as the prime minister in the new administration.
In countries with semi-presidential constitutions, I can think of only one other case where the outgoing president has served as prime minister under the incoming president and that is Timor-Leste. Here, Jose Gusmao was president from May 2002 to May 2007 before serving as prime minister since this time. Indeed, the East Timor case is particularly unusual because Jose Ramos-Horta was Prime Minister from 2006-2007 before being elected President. This is the only case I know where in a semi-presidential democracy the two most senior incumbents have swapped offices.
There are a small number of examples of prime ministers being elected as president without a gap in between the two offices. Examples include Mario Soares in Portugal, though technically there was a small gap here as he resigned as PM in November 1985 and took office as president in March 1986; and Janez Drnovsek, who was PM in Slovenia from November 2000 until December 2002 when he took up his post as president. Obviously, there are many more cases where prime ministers have been elected president with a gap in between. Examples include Stipe Mesic in Croatia, Jacques Chirac in France, Rolandas Paksas in Lithuania, Nambaryn Enkhbayar in Mongolia, and Victor Yushchenko in Ukraine.
It is hardly surprising that there are more examples of former PMs being elected as president, after all usually the presidency is the only position that is more important than the prime ministership and so prime ministers often have a natural ambition to try to win the top office. Indeed, there was a book published in France in the 1980s that identified a prime ministerial Oedipus complex – as soon as the prime minister was appointed they wanted to kill the father (politically, of course) and assume the top job for themselves. (Pierre Servent, Oedipe à Matignon, Paris: Balland, 1989).
If anyone can think of any other examples of presidents and prime ministers swapping positions, or of outgoing presidents moving to the post of prime minister, or vice versa, then please leave a comment.