upgrade_to_friend_effect

upgrade_to_friend_effect = {
	#If nemesis, change to rival
	if = {
		limit = { has_relation_nemesis = $CHARACTER$ }
		remove_relation_nemesis = $CHARACTER$
		if = {
			limit = {
				exists = scope:relationship_reason_involved_character
			}
			set_relation_rival = {
				target = $CHARACTER$
				reason = $REASON$
				involved_character = scope:relationship_reason_involved_character
			}
		}
		else = {
			set_relation_rival = {
				target = $CHARACTER$
				reason = $REASON$
			}
		}
	}
	#If rival, end rivalry.
	else_if = {
		limit = { has_relation_rival = $CHARACTER$ }
		remove_relation_rival = $CHARACTER$
	}
	# If not, look at upgrading from friend to best friend
	else_if = {
		limit = {
			has_relation_friend = $CHARACTER$
		}
		if = {
			limit = {
				exists = scope:relationship_reason_involved_character
			}
			set_relation_best_friend = {
				target = $CHARACTER$
				reason = $REASON$
				copy_reason = friend
				involved_character = scope:relationship_reason_involved_character
			}
		}
		else = {
			set_relation_best_friend = {
				target = $CHARACTER$
				copy_reason = friend
				reason = $REASON$
			}
		}
	}
	# If not, look at upgrading from nothing to friend
	else_if = {
		limit = {
			NOT = { has_relation_friend = $CHARACTER$ }
		}
		if = {
			limit = {
				exists = scope:relationship_reason_involved_character
			}
			set_relation_friend = {
				target = $CHARACTER$
				reason = $REASON$
				involved_character = scope:relationship_reason_involved_character
			}
		}
		else = {
			set_relation_friend = {
				target = $CHARACTER$
				reason = $REASON$
			}
		}
	}
}

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