Why did Goddess Adi Parashakti not return to her original form when she took the form of Lord Shiva's wife Goddess Parvati? What happens to her?
First of all, according to Shakta traditions, Adi Shakti exists before, during and after the existence of everything else so her taking the form of Sati or Parvati is somewhat similar to Lord Vishnu appearing as one of his Avatars.
But even if we assume that the goddess left her form and transformed into Parvati, she does return to her original form many times as evidenced by the following episodes from Devi Bhagwatam.
Devi Bhagwat Puran Book 5 Chapter 23 mentions:
1-7. Vyâsa said :-- O King! When the tormented Devas praised thus, the Devî created from Her body another supremely beautiful form. This created form, the Ambikâ Devî, became known in all the worlds as Kaus’ikî, as She came out of the physical sheath of the Devî Parvatî. When Kaus’ikî was created out of the body of Parvatî, the Parvatî’s body became transformed and turned out into a black colour and became known as KâIikâ. Her terrible black appearance, when beheld, increases the terror even of the Daityas. O King! This Devî is now become known in this world as Kâlarâtri, the night of destruction, at the end of the world, identified with Durgâ, the Fulfiller of all the desires. The Ambikâ Devî, then, began to look splendid, decked with various ornaments; Her beautiful form began to look very lovely. The Devî Ambikâ then smiled a little and said, “Better be fearless; I will slay just now your enemies. It is My incumbent duty to carry out your purposes; I will therefore slay in battle Nis’umbha and others for the sake of your happiness.”
Again in Book 7 Chapter 28 Parvati transforms into Shatakshi (one with a thousand eyes) also called Shakambhari who further gets the name of Durga after killing the demon Durgam:
There, Goddess Parvati went to Himalayas where Gods were praying to her. Gods informed her about the drought condition over the earth. Seeing the dreadful condition of the earth, she created innumerable eyes within Her body and became visible. That Upholder of the Universe, showed Her form and began to shed waters from Her eyes. For nine nights continuously, the heavy rains poured down out of the waters flowing from Her eyes. Seeing the misery of all the people, out of pity, She showered incessantly tears from Her eyes; and all the people and medicines were satisfied. What more than this, out of those tears, the rivers began to flow. The Gods that remained hidden in the mountain caves, now came out. Then the sages, united with the Gods, began to praise and sing hymns to the Goddess. Then, Shatakshi Devi transformed her appearance into a wonderful form, her eight hands held foods like grains, cereals, vegetables, greens, fruits, meat and other herbs, she wore a beautiful garment, this new form of Goddess is known as Shakambhari. Goddess Parvati transformed her appearance. Now she looked ferocious in her new appearance. She was well equipped with all sorts of lethal weapons and was mounted on a lion. She thundered loudly and challenged Durgamasur.
Furthermore, in the same episode, there is also the appearance of many other goddesses from Shakambhari Devi which shows that she is no different from the Adi Shakti:
At this moment, came out of the body of the Devi, the principal Shaktis (forces incarnate) named Kali, Tarini, Tripura-Sundari, Bhuvaneshwari, Bhairavi, Chinnamasta, Dhumavati, Bagalamukhi, Matangi and Kamalatmika. The Navadurgas Shailaputri, Brahmacharini, Chandraghanta, Kushmanda, Skandamata, Katyayani, Kalratri, Mahagauri, Siddhidatri and Matrikas Brahmani, Vaishnavi, Maheshwari, Kaumari, Indrani, Varahi, Narasimhi, Shivadooti, Chamundai also came out of the Goddess.
Then, Devi, Shakti, Adi Shakti, Parameshwari, Gauri, Jagadamba, Maha Shakti, Bhagawati, Bhadrakali, Chandika, Kaushiki, Mahakali, Tridevi, Mahadevi, Ambika, Jagatjanani, Mateshwari, Uma, Narayani, Singhavahini, Aparajita, Vyaghravahini, Bhavani, Yudhadevi, Yogmaya, Mahamaya, Sherawali, Vindhyavasini, Rajarajeshwari, Kalika, Gayatri, Shaambhavi, Bhramari, Jumadi, Akhilandeshwari, Kamakshi, Mahadurga, Prathyangira, Meenakshi, Ramchandi, Ishwari, Gatral, Bahuchara, Hingalaj, Nagbai, Varaki, Dhavdi, Sadhi, Hadkai, Vihat, Vaishnodevi, Santoshi, Mogal, Pithad, Meldi, Umiya, Momai, Amba, Khodiyar, Jagadhatri, Modheshwari, Guhyakali, Shitala and Raktachamunda joined in the war against Durgamasur. Jambhini, Mohini, Tripur and Shodashi came out the Goddess too.
Some Yoginis named Aditi, Agneyei, Ajita, Aparna, Bhayankari, Bhimachandi, Chandi, Damani, Dhriti, Gandhari, Ganga, Jaya, Kapalini, Kauberi, Medha, Rati, Raudri, Rudrani, Saraswati, Sarvamangala, Shankari, Shanti, Siddhida, Swaha, Swadha, Varuni, Vijaya, Vrishaba Vahana and Yakshini came out from the Goddess as well. Also Mahasaraswati, Chandavigrah, Savitri and Trishi came out. Some other Goddesses named Mrityu Devi, Saranyu, Vayu Devi and Ganeshi came out of the Goddess successfully.
69-73. The Devas said :-- “O Auspicious One! Thou art the only Cause of this Illusion of this world, presenting an unreal appearance. So Thou art the Lady of all the beings. So, Obeisance to Thee, the S’âkambharî! Hundred-eyed! O Auspicious One! Thou art sung in all the Upanisadas; The Destroyer of the Durgama Asura! We bow down to Thee, the Lord of Mâyâ, the Dweller in the five sheaths Anna, Rasa, etc. We meditate upon Thee, the Lady of the universe, as demonstrated by Pranava Aum, whom the chief Munis meditate with their Nirvikalpa hearts. Thou art the Mother of the endless crores of universe! Thou assumest the Divine Bodies at times for our welfare! Thou art the Mother of Brahmâ, Visnu and others; we bow down to Thee with all our heart.
When Adi Parashakti took the form of Parvati,sometimes she had showed her original form like when she had killed Mahishasura in the form of Durga,she had killed Sumbha and Nisumbha in the form of Kali,she had killed Arunasura in the form of Bhramari and once she also came in the form of Akhilendeshwari to pacify Lord Shiva.