Changing of the Guard
From Halopedia, the Halo wiki
The Brute Honor Guards took the place of the Elites as the protectors of the High Prophets after the murder of the High Prophet of Regret. This "Changing of the Guard" was the beginning of the Civil War of the Covenant, as the Elites were incensed that their role of protecting the Prophets, which they had faithfully executed for millenia, had been now delegated by these new upstarts and genetic rivals. The Changing of the Guard occured in the beginning of the campaign level The Sacred Icon. The swap takes place during the opening cutscene for and Brutes can be seen taking the ceremonial staves and helmets from former Elite Honor Guards.