The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
21. (The initial statement identifies the individual soul and the highest Self) because the soul when it will depart (from the body) is such (i.e. one with the highest Self); thus Audulomi thinks.
The individual soul which is inquinated by the contact with its different limiting adjuncts, viz. body, senses, and mind (mano-buddhi), attains through the instrumentality of knowledge, meditation, and so on, a state of complete serenity, and thus enables itself, when passing at some future time out of the body, to become one with the highest Self; hence the initial statement in which it is represented as non-different from the highest Self. This is the opinion of the teacher Audulomi.--Thus Scripture says, 'That serene being arising from this body appears in its own form as soon as it has approached the highest light' (Kh. Up. VIII, 12, 3).--In another place Scripture intimates, by means of the simile of the rivers, that name and form abide in the individual soul, 'As
the flowing rivers disappear in the sea, having lost their name and their form, thus a wise man freed from name and form goes to the divine Person who is greater than the great' (Mu. Up. III, 2, 8). I.e. as the rivers losing the names and forms abiding in them disappear in the sea, so the individual soul also losing the name and form abiding in it becomes united with the highest person. That the latter half of the passage has the meaning here assigned to it, follows from the parallelism which we must assume to exist between the two members of the comparison 1.
278:1 Bhâmatî: The individual soul is absolutely different from the highest Self; it is inquinated by the contact with its different limiting adjuncts. But it is spoken of, in the Upanishad, as non-different from the highest Self because after having purified itself by means of knowledge and meditation it may pass out of the body and become one with the highest Self. The text of the Upanishad thus transfers a future state of non-difference to that time when difference actually exists. Compare the saying of the Pâñkarâtrikas: 'Up to the moment of emancipation being reached the soul and the highest Self are different. But the emancipated soul is no longer different from the highest Self, since there is no further cause of difference.'--The technical name of the doctrine advocated by Audulomi is satyabhedavâda.