Seán Manchester (from his writings) on shapeshifting:
The Book of Enoch (also 1 Enoch) is a pseudepigraphic work ascribed to Enoch, the great-grandfather of Noah and son of Jared (Genesis 5: 18). While this book today is non-canonical in most Christian Churches, it was explicitly quoted in the New Testament (Letter of Jude 1: 14-15) and by many of the early Church Fathers. The Ethiopian Orthodox Church to this day regards it to be canonical. It is wholly extant only in the Ge'ez language, with Aramaic fragments from the Dead Sea Scrolls and a few Greek and Latin fragments. There is no consensus among Western scholars about the original language: some propose Aramaic, others Hebrew, while the probable thesis according to E Isaac is that 1 Enoch, as Daniel, was composed partially in Aramaic and partially in Hebrew. Ethiopian scholars hold that Ge'ez is the language of the original from which the Greek and Aramaic copies were made, pointing out that it is the only language in which the complete text has been found.
The Book of Enoch describes the fall of the Watchers, the angels who fathered the Nephilim. The fallen angels went to Enoch to intercede on their behalf with God after he declared to them their doom. The remainder of the book describes Enoch's visit to Heaven in the form of a vision, and his revelations. The antediluvian patriarch Enoch according to Genesis"walked with God and was seen no more, because God took him." This walking with God was naturally understood to refer to special revelations made to the patriarch, and this, together with the mystery surrounding his departure from the world, made Enoch's name an apt one for the purposes of apocalyptic writers. In consequence there arose a literature attributed to him.
It influenced not only later Jewish apocrypha, but has left its imprint on the New Testament and the works of the early Fathers. The canonical Epistle of St Jude, in verses 14, 15, explicitly quotes from the Book of Enoch; the citation is found in the Ethiopic version in verses 9 and 4 of the first chapter. There are probable traces of the Enoch literature in other portions of the New Testament. Passing to the patristic writers, the Book of Enoch enjoyed a high esteem among them, mainly owing to the quotation in Jude. The so-called Epistle of Barnabas twice cites Enoch as Scripture. Clement of Alexandria, Tertullian, Origen, and even St Augustine suppose the work to be a genuine one of the patriarch. But in the fourth century the Enoch writings lost credit and ceased to be quoted. After an allusion by an author of the beginning of the ninth century, they disappear from view. So great was the oblivion into which they fell that only scanty fragments of Greek and Latin versions were preserved in the West. The complete text was thought to have perished when it was discovered in two Ethiopic manuscripts in Abyssinia, by the traveler Bruce in 1773. Since, several more copies in the same language have been brought to light. Recently a large Greek fragment comprising chapters i-xxxii was unearthed at Akhmîn in Egypt.
Scholars agree that the Book of Enoch was originally composed either in Hebrew or Aramaic, and that the Ethiopic version was derived from a Greek one. A comparison of the Ethiopic text with the Akhmîn Greek fragment proves that the former is in general a trustworthy translation. The work is a compilation, and its component parts were written in Palestine by Jews of the orthodox Hasidic or Pharisaic schools. Its composite character appears clearly from the palpable differences in eschatology, in the views of the origin of sin and of the character and importance of the Messias found in portions otherwise marked off from each other by diversities of subject. Critics agree that the oldest portions are those included in chapters i-xxxvi and (broadly speaking) lxxi-civ.
Shapeshifting is a common theme in mythology and folklore, as well as in science fiction and fantasy. In its broadest sense, it is when a being undergoes a transformation. Commonly the transformation is purposeful, and not a curse or spell. In some folklore once the shapeshifter transformed, it began to get harder and harder to change back to ones original form. Vampires and werewolves are somewhat similar. Vampires, in older pieces of mythology and folklore, were thought to be able to transform into a wolf or a bat, thus giving the vampire bat its name. Most shapeshifters change into an animal, they were believed to only be able to change into an animal, or person that they had seen.
The most important aspect of shape-shifting, thematically, is whether the transformation is voluntary. Circe transforms intruders to her island into swine, whereas Ged, in A Wizard of Earthsea, becomes a hawk to escape an evil wizard's stronghold. A werewolf's transformation, driven by internal forces, is as hideous as that which Circe enforces, and when Minerva transforms Cornix into a crow, Ovid put into Cornix's mouth that"the virgin goddess feels pity for a virgin and she helped me" because her new form enabled her to escape rape at Neptune's hands. When a form is taken on involuntarily, the thematic effect is one of confinement and restraint; the person is bound to the new form. In extreme cases, such as petrifaction, the character is entirely disabled. Voluntary forms, on the other hand, are means of escape and liberation; even when the form is not undertaken to effect a literal escape, the abilities specific to the form, or the disguise afforded by it, allow the character to act in a manner previously impossible.
I could not opine as to whether shapeshifting was taught to men prior to the Flood. What I will say is that the phenomenon is something familiar to demons.