My 'struggles' with the movie
Let me just start off by saying I think this movie is less than a 5, and I'm only willing to give it a 4 for the good performance Olivia gave us. Now, to the point...
Theory #1
While watching this, I kept on thinking this was not actually a possession, or a conscience swap and more of a PTSD. Hanna simply died, the thought of losing her child and the guilt she was feeling was unbearable and she wasn't in a good condition to begin with. Sam survived, knowing her mother was gone. Sam, both guilty over how she treated her mother and traumatized over the accident, shows signs of Dissociative identity disorder (DID). I know it's usually more than 2 personalities, but it was the only "logical" explanation I could come up with. this was especially going through my mind the first time she "became" Sam.
Theory #2
It really was Hanna and Sam both in the same body, and since the energy of two souls (which are energy ) is too much for one body to handle, they were fighting for a place and Hanna let go...letting Sam take over and be the Id. So the ending was just that, Sam wanting to tell her mom, who was still, as she put it, around there somewhere, that she loved her and cared about her.
Theory #3
Sam thinks she's her mom, so her mind strongly pretends she is her. This will not explain the many things Sam knows about her parents, but those are things that Sam could have known, so we would never know! This was inspired by the movie: The Cow http://www.imdb.com/title/tt0064356/ about a man who thought he was a Cow. Again, I think this is somewhat the same as the PTSD, but maybe even more severe since this shows Sam is seriously crazy. (whereas someone with DID is not "crazy", just has a personality disorder which is treatable by meds. )
Theory #4
What most people around here seem to have taken from the movie. It really happened, two people lived in the same body. The transition happened when they were both dying, their consciences got confused. Maybe one of them had to go, maybe both...somehow, they were both pulled into Sam's mind. In the end, only one could stay around and it seemed like it was Sam who was there... but like most other people say, it could have been Hanna, pretending to be Sam for everyone's sake.
Good movie? It could have been.