convert_player_realm_from_old_culture_to_new_effect

convert_player_realm_from_old_culture_to_new_effect = {
	# For localization
	$OLD_CULTURE$ = { save_scope_as = old_culture }
	$NEW_CULTURE$ = { save_scope_as = new_culture }

	custom_tooltip = global_culture.custom.change_culture
	hidden_effect = {
		# Compile a list of everyone who will flip to the new culture.
		every_vassal_or_below = {
			limit = {
				culture = $OLD_CULTURE$
			}
			add_to_list = characters_to_convert
			every_courtier = {
				limit = {
					culture = $OLD_CULTURE$
				}
				add_to_list = characters_to_convert
			}
		}
		every_courtier = {
			limit = {
				culture = $OLD_CULTURE$
			}
			add_to_list = characters_to_convert
		}

		# Flip the player to the new culture.
		set_culture = $NEW_CULTURE$
		add_character_flag = converted_culture_this_lifetime

		# Flip their courtiers/vassals with a custom description.
		every_in_list = {
			list = characters_to_convert
			set_culture = $NEW_CULTURE$
		}

		# Then flip all their counties.
		every_sub_realm_county = {
			limit = {
				culture = $OLD_CULTURE$
			}
			set_county_culture = $NEW_CULTURE$
		}
	}
}

As written in game/common/scripted_effects/00_culture_effects.txt.