In the Mahabharata Mahadeva takes Vyuptakesha form as described in this chapter of Drona Parva where story of Tripura Dahana is told in short. Mahadeva takes bald headed form of a child to destroy ego of Indra:
When, however, the three cities came together in the firmament, the lord Mahadeva. pierced them with that terrible shaft of his, consisting of three knots. The Danavas were unable to gaze at that shafts inspired with Yuga-fire and composed of Vishnu and Soma. While the triple city commenced to burn, the goddess Parvati repaired thither to behold the sight. She had then on her lap, a child having a bald head with five clumps of hair on it. The goddess asked the deities as to who that child was. Sakra, through ill-feeling endeavoured to strike that child with his thunderbolt. The divine lord Mahadeva, smiling, quickly paralysed the arm of the enraged Sakra. Then god Sakra, with his arm paralysed accompanied by all the celestials, speedily repaired to the lord Brahma of unfading glory. Bowing unto him with their heads, they addressed Brahma with joined hands and said, 'Some wonderful creature, O Brahma, lying on the lap of Parvati, in the form of a child, was behold by us but not saluted. We have all been vanquished by him. We, therefore, desire to ask thee as to who he may be. Indeed, that boy, without fighting, hath with the greatest ease vanquished us all with Purandara at our head.' Hearing these words of theirs, Brahma. that foremost of all persons, acquainted with Brahma, reflected for a moment and understood that boy of immeasurable energy to be none else than the divine Sambhu, Addressing then, those foremost of celestials with Sakra at their head, Brahma said, 'That child is the divine Hara the Lord of the entire mobile and immobile universe. There is nothing superior to Maheswara. That Being of immeasurable splendour who was beheld by you all with Uma, that divine lord, had assumed the form of a child for Uma's sake. Let us all go unto him. That divine and illustrious one is the Supreme Lord of the world. Ye gods, ye could not recognise that master of the universe.' Then all the gods with the Grandsire repaired to that child, endued with the effulgence of the morning sun. Beholding Maheswara, and knowing that he was the Supreme Being, the Grandsire Brahma thus adored him: Thou art Sacrifice, O lord, thou art the stay and refuge of the universe. Thou art Bhava, thou art Mahadeva, thou art the abode (of all things), and thou art the highest refuge. This whole universe with its mobile and immobile creatures, is pervaded by thee. O holy one, O lord of the past and the future, O lord of the world, O protector of the universe, let Sakra, afflicted with thy wrath, have thy grace.'
"Vyasa continued, 'Hearing these words of the lotus-born Brahma, Maheswara became gratified. Desirous of extending his grace, he laughed aloud. The celestials then gratified (with praise) both Uma and Rudra. The arm of the thunder-wielding Sakra re-got its natural state. That foremost one of all the gods, that destroyer of Daksha's sacrifice, that divine lord having the bull for his sign, became gratified with the gods. He is Rudra he is Siva, he is Agni, he is everything, and he hath knowledge of everything....."
If we go by intrepretations of Acharya. Three Bhāṣyams on Śrī Rudram exists.
- Sāyana Bhāṣya
- Abhinava Śaṅkara Bhāṣya
- Bhaṭṭabhāskara Bhāṣya
Among all The Abhinava Śaṅkara Bhāṣya is considered to be the best.
Sāyana Bhāṣya states that (Anandashram Samskrita Granthavali, ASS_002 contains both Sāyana Bhāṣya and Bhaṭṭabhāskara Bhāṣya)
मुण्डितकेशो व्युप्तकेशः । यत्यादिवेषेण मुण्डितकेशत्वम् ।
Abhinava Śaṅkara Bhāṣya states that
व्युप्ताः मुण्डिताः केशाः यस्य सः व्युप्तकेशः परमहंसपरिव्राजकः ।
Both the Bhāṣyas say the same up to here. That is one with shaven head and he who is a yatī. (Here Namaskarams to Kapardi and Vyuptakesa in singular form).
Abhinava Śaṅkara further explains as
'अथ परिव्राडेकशाटी शिष्यपरिवृतो मुण्डोदरपात्री' 'ज्ञानशिखिनः
ज्ञानयज्ञोपवीतिनः' इत्यादिश्रुतेः ; तेन
दत्तात्रेयदुर्वासः प्रभृतयः जाबालोपनिषदादिप्रसिद्धाः कलियुगे च 'चतुर्भिः सहितः शिष्यैः
सविताननः' इत्यादि पुराणप्रसिद्धपरमेश्वरावतारभावाः शंकराचार्यस्वामिनः
तद्रूपायेत्यर्थः । अनेन
'ब्रह्माधिपतिर्ब्रह्मणोऽधिपतिः' इति श्रुतिप्रसिद्धं ब्रह्मविद्यासंप्रदायप्रवर्तकत्वमुक्तम् ।
A translation in accordance with the above three commentaries (Sayana, Bhattabhaskara and Abhinava Sankara) from the Kanchi Kamakoti Website gives the explanation as
‘Kapardi’ means one who has adorable matted locks. This serves to
remind of the greatness that a part of these matted locks became
‘Virabhadra’, who destroyed Dakska’s yagna. Sruti states the
characteristics of Sanyasis, who are Paramahamsas: ‘They sport shaven
heads; they wear saffron coloured clothes; their hair and yagnopavita
is Atma gnana’. Dattatreya, Durvasa et al were such Gnanis in previous
yugas. In this Kaliyuga, as per the statement, ‘चतुर्भिस्सहितः
शिष्यैश्शङ्करोवतरिष्यति’ (Sri Parameswara is going to incarnate with
four disciples), Sri Sankara Bhagavatpada Acharya Swamigal,
incarnation of Sri Parameswara, also had these characteristics. The
term ‘Vyptakesaaya cha’ signifies that it is Sri Parameswara alone who
shines in these forms.
In the Sruti passage, ‘ब्रह्माधिपतिर्ब्रह्मणोधिपतिः’, Sri Parameswara
is described as the pioneer of the tradition of Brahmavidya (Knowledge
of Brahman). It is clear that he took the forms of Sankaracharya et al
and became pioneers of the tradition of Brahmavidya.
So it is Śiva as Jagadguru Śrī Ādi Śaṅkarācārya that is referred by व्युप्तकेशाय here.
Hope this helps you.