As it states in the text for the stratagem:
Use this Stratagem in your Command phase. Select one EMPEROR’S CHOSEN unit from your army, then select one other shield host’s fighting style. Until the start of your next Command phase, the models in that unit have the traits of that shield host fighting style instead of the traits of the Magna Imperator fighting style.
The only thing you change in the unit's fighting style is the traits of the shield host so they keep Rendax as their favorite.