There is a really cool short story about this. I can't remember the name of it for the life of me, but it is in a collection of H.P. Lovecraft tribute stories. In this story, there is a couple and the woman becomes obsessed with astral travel -- always reading books and leaving them around the house, and she begins to neglect her boyfriend. Eventually he decides to read the books and see what it is she has been up to. He learns how to astral project, and in the astral plane he sees that his girlfriend has found her soulmate and they are in love. The girlfriend went through all of this trouble to trick the boyfriend into leaving his body so that the soulmate could possess it. And the boyfriend is left body-less and pursued by some unnameable horror.
BUT this is highly, HIGHLY unlikely because our souls are attached to our body by a sort of cord, which enables us to travel wherever we want and ensure that we don't get lost and return safely. This cord is believed to be severed only during death; so no, you can't switch bodies. Hypothetically you could possess someone, but I think this is only possible if you are already parted from your body (dead) because to be attached to one body and controlling another might be too large a division of consciousness to really work.