don 't think so, the deku tree in WW told you that the koroks were once in human form, but after the flood occurd they had to adjust their form to life on.
i don 't know that for sure about leaving the forest. at the ending of oot the dance at lon lon ranch. my theory is that kokiri adjust their form in drastic envoirments, well its just a theory i can't prove it.
as far as i know, if you have a fairy and you are in lost woods, you will be able to find your way back(don 't know but someone said that in the game) and if somebody gets lost they become a stalfos, well the blond kokiri girl said that if you gave the odd potion in exchange for poachers saw...