CC Madhya 1.208
dainya chāḍa, tomāra dainye phāṭe mora mana
Actually this is Śrī Caitanya Mahāprabhu’s initiation of Dabira Khāsa and Sākara Mallika. They approached the Lord with all humility, and the Lord accepted them as old servants, as eternal servants, and He changed their names. It is to be understood from this that it is essential for a disciple to change his name after initiation.
tan nāma-karaṇaṁ caiva vaiṣṇavatvam ihocyate
“After initiation, the disciple’s name must be changed to indicate that he is a servant of Lord Viṣṇu. The disciple should also immediately begin marking his body with tilaka (ūrdhva-puṇḍra), especially his forehead. These are spiritual marks, symptoms of a perfect Vaiṣṇava.” This is a verse from the Padma Purāṇa, Uttara-khaṇḍa. A member of the sahajiyā-sampradāya does not change his name; therefore he cannot be accepted as a Gauḍīya Vaiṣṇava. If a person does not change his name after initiation, it is to be understood that he will continue in his bodily conception of life.