When I was there in March 2007 the change took place at 15:00. A contingent from the Navy marched up the street into the piazza led by a band from La Guardia di Finanza, the Customs Police. The current guard contingent from the Air Force marched out and, once in fromation on the piazza, sang a stirring rendition fo the nation anthem.
The sergeants of the guards from both services then relieved and posted the new sentries at the entrance.
The sailors then sang the national anthem before marching into the palazzo. It was very enjoyable to see the pomp and an old military man like me always enjoys seeing fine troops on parade.