I've heard of cases like these. People get major organ transplants and develop traits they've never had before. But the thing is that the entire body is tied together. Your own body becomes part of your subconscious and conscious routine, so when you donate an organ to someone, they get a part of you and your subconscious.

If you research into those stories, the people who look into why they act like that usually go to the family of the donors and find out that the trait they have is the same trait the person who donated once had. It just goes to show you what amazing and complex bodies we have.

And no the person doesn't change completely, they just gain a trait or two. Its kinda like upgrading except not in the sense that you improve yourself.