Though the Lancelot section of the cycle has him converted by Joseph himself, the Estoire del Saint Graal section, written after the Vulgate Lancelot as a prequel, states that Joseph's son Josephus converted him.
There is a discrepancy in the narrative regarding Lancelot's conversion, as the Estoire del Saint Graal section presents it as an event initiated by Josephus, while the Vulgate Lancelot portrays Joseph as the one who underwent this transformation.