[1] Cabinet reshuffles happen in parliamentary systems for a variety of reasons.
Periodically, smaller reshuffles are needed to replace ministers who have resigned, retired or died.
Similarly, prime ministers entering office from the same party as the previous one might appoint a very different ministry than their predecessor to reflect a change in policies and priorities; an example is Gordon Brown's government, formed on 28 June 2007 after the departure of Tony Blair the previous day.
In such systems there is a far larger pool of viable candidates to choose a cabinet from.
On an individual basis, however, U.S. cabinet members will occasionally change departments—for example, Federico Peña served as Secretary of Transportation from 1993 until 1997 in the Clinton administration and was appointed as Secretary of Energy directly afterwards, serving until 1998.