downgrade_to_rival_effect

downgrade_to_rival_effect = {
	#If best friend, change to friend
	if = {
		limit = {
			has_relation_best_friend = $CHARACTER$
		}
		remove_relation_best_friend = $CHARACTER$
		set_relation_friend = { reason = friend_downgrade_best_friends target = $CHARACTER$ }
	}
	#If friend, end friendship.
	else_if = {
		limit = {
			has_relation_friend = $CHARACTER$
		}
		remove_relation_friend = $CHARACTER$
	}
	# If not, look at upgrading from rival to nemesis
	else_if = {
		limit = {
			has_relation_rival = $CHARACTER$
			NOT = { has_relation_nemesis = $CHARACTER$ }
		}
		set_relation_nemesis = {
			target = $CHARACTER$
			copy_reason = rival
			reason = nemesis_downgrade_from_rival
		}
	}
	# If not, look at upgrading from nothing to rival
	else_if = {
		limit = {
			NOT = { has_relation_nemesis = $CHARACTER$ }
		}
		set_relation_rival = {
			target = $CHARACTER$
			reason = $REASON$
		}
	}
}

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