When we decide to incarnate 'as' a body, we seem to take on a particular form, which btw is constantly changing, and gradually steer the manifestation of that body in various directions of growth and change. This is all done in the mind.
It is possible for some individuals with greater awareness to shift the form of the body from one state to another more readily utilizing mind power and miracles, for example. Health can turn into sickness, and sickness into health. These are shifts in physicality which you would think are not really shifts of a singular globular 'blob' of matter, but it really is.
Similarly, all shapeshifter are capable of morphing into any number of forms, even beyond the human form.
Now, the interesting thing also with shapeshifter is that there is a 'pool' or 'ocean' of unifies, blended-together shape shifters who are in what you could call a non-manifest or non-specific form. This is very nicely illustrated in the Star Trek Deep Space Nine series btw where Odo, a significant character, is a shapeshifter who can become any form and can return to a lake filled with other shape shifters who are all blended together. On a physical level this is representative, symbolically, of what we are capable of, and in fact we are also capable of these physical shifts.
The point to be made here, is that it is natural and normal for us to co-exist in a unified pool of 'oneness' where we are not any specific form and where we are all the same, sharing everything with each other. This is our home. The Spirit of God.
So really we are not locked into forms, but we come to believe we are, and act like our body is rigid and changes slowly and is very hard to adjust at will. But we can. And this ability can strengthen as we transcend the body in its limiting awareness, and return to an awareness that we are spiritual shape-shifters, or rather, shape-transcendent. Form is not who we are and it is not our true identity, or our natural home. It is just something we become imprisoned in for a while.