Coming to power offers a natural time to re-shuffle the cabinet. There may have been members of your shadow cabinet who retired at the election, and you may wish to promote your allies into positions of power, as having just won an election you will be popular within your party. You may also want to re-organise government, consistent with your political views.
We haven't had a clean takeover of power since '97 when Tony Blair became PM, as in 2010 the coalition negotiations meant that Lib Dems had to be given a certain number of positions of government.
For the Labour party there has been a further consideration: in opposition the shadow cabinet is elected. While in government the leader can appoint without reference to the party. This means that the Labour party leader can't pick his or her cabinet freely until they come to power.
Looking back to '97 most of the major roles were unchanged: Chancellor, Home Affairs and Foreign Affairs were taken by their shadows. Defence, Education-and-Employment, Health were similarly unchanged.
Others were shuffled. Jack Cunningham, who had shadowed National Heritage, move to Agriculture Fisheries and Food. Gavin Strang (who had been at MAFF) moved to Transport, while Andrew Smith who had been at Transport was demoted to a non-cabinet position. (Andrew Smith had been seen as a Left winger, not a Blairite) The National Heritage role was eliminated, as was the Dept. for the Citizen's Charter. Derek Foster, who had shadowed the Citizen's Charter was also demoted to a non-cabinet role.
In general, most Prime Ministers will use the change of Government to reshuffle their Cabinet, but are tend not to make major changes at the top in a team that has just won an election.