替わる can work as a transitive verb when meaning "to be changed/replaced a roll/position/situation".
e.g.
運転を替わる
電話を替わる
担当を替わる
議長を替わる
役目を替わる
The transitive verb 替える often has a forced nuance, so it's considered that the intransitive verb 替わる is used to soften the nuance. In fact, it's often used when the subject is the speaker themselves.