Today, story of ichaadhari nagas and naagins are widely used in tv serials. In the content of one serial I read that this concept of shape-shifting snakes exists in Hindu scriptures. But I failed to find this concept in scriptures. Does this concept exists in Hinduism? Are snakes like Takshak, Vasuki, Adishesha etc. shape shifting snakes.
Maybe not exactly what you are thinking of, but in the Mahābhārata story of Nala and Damayantī, the snake Karkoṭaka bites Nala in order to temporarily disguise Nala with a different appearance, and Karkoṭaka himself takes on the form of Nala for the duration. This is a kind of shape-shifting. (MBh 3.63.12: svarūpadhāriṇaṃ nāgaṃ dadarśa ca mahīpatiḥ)