The transformation that takes place within the Juniper Tree I believe is different from the other transformation stories based on two main facts. The first is that the boy is dead before he becomes a bird in the Juniper Tree. In the other stories the boys that get transformed are always alive and then transformed. The fact that the boy is dead and then becomes a bird in the Juniper Tree leads me to believe that the bird is meant to be the physical manifestation of the soul/angel/ ghost or whatever else you would like to call it. This leads me to the second difference, which is that the brother does all the work to be converted back into human form. The sister does her part at the beginning by burying the bones, but that aside, the boy does all the work. In the other brothers into birds stories the sister has to go through some tumultuous trial, usually over the course of several years, to finally turn the brothers back into humans. In the Juniper Tree it seems as if the boy takes one to two days max before he is reborn as his old self.