People like Shem, Melchizedek, Enoch, etc., are associated with Christ, and foreshadow Christ in certain ways. But they were their own people, and they were not incarnations of Jesus in disguise.
Melchizedek is listed as a saint by all the ancient churches. His Western feast day is August 26, and his Eastern one is coming up on May 22. He was an ancestor of Jesus.
That said, Jesus did have non-incarnate apparitions (usually called “theophany” or “Christophany”) in Old Testament times (such as the “one like a son of man” in the Book of Daniel), and there were times when the angels who appeared to give messages from God have been closely associated with revelations pointing to Jesus. There are some.people.who believe that when Abraham met up with Melchizedek, he may have actually met a Christophany of Jesus, taking Melchizedek’s place. That is an allowable ancient opinion.
Angels sometimes take on the appearance of a particular human, or pretend to be human (although not directly lying). But they are not incarnate or reincarnating when they do this; they are spirits.