In this paper, I analyze a previously unpublished Leibniz text from the early 1700s. I give it the title "On Unities and Transmigration" since it contains an outline of his doctrine of unities and an examination of the doctrine of transmigration. The text is valuable because in it Leibniz considers three very specific versions of transmigration that he does not address elsewhere in his writings; these are (1) where a soul is released by the destruction of its body and is then free to pass into another body, (2) where souls are exchanged without any destruction of bodies, and (3) where human souls (minds) are exchanged, again without any destruction of bodies. I show that when tackling these three versions of transmigration in "On Unities and Transmigration," Leibniz develops a series of objections that are not found anywhere else in his published writings, despite his lifelong opposition to the doctrine of transmigration. This paper is completed by two appendices, the first of which presents the previously unpublished "On Unities and Transmigration" text in full, in the original French (with all deletions indicated), while the second presents its English translation.