How do you change from spartan to elite in Halo 3?

The elites are on your side because since they did a bad job of protecting one of the Prophets they made the Brutes their body guards and started killing all the Elites, this made the Elites mad because ever since the Covenant was founded the Elites had been the royal guard. And since they have a huge dislike for the Brutes they join up with the humans to wipe out the Covenant. Oh and also they found out that the Prophets want to activate the Halo rings which will wipe out all living life in the Galaxy and that the "Great Journey" is all a lie.