Changing of the Guard

The Changing of the Guard was a major event of the Covenant and it contributed greatly to the start of the Great Schism, also known as the Covenant Civil War.

After the assassination of the High Prophet of Regret, the Brutes forcibly took the place of the Elites as the protectors of the High Prophets, after the Elite Honor Guards were unable to protect Regret. As a result, the Elites were incensed that their role of protecting the Prophets, which they had faithfully executed for a millennium, had been now delegated by these new upstarts and genetic rivals.

The Changing of the Guard occurred in the cutscene at the beginning of the Halo 2 level The Sacred Icon (Level). The Brutes can be seen taking the ceremonial staves and helmets from former Elite Honor Guards.

Trivia
The High Prophet of Truth actually indistinctly entitled the event as "Exchange of Hats"