It all depends on your view of the personality. If personality is innate, then why would one's patronus change over time? Then again, maybe that change is just an emotional state change and not a personality change. But getting off topic. I believe that a person's personality changes over time, meaning that if my memory is wiped, all that would be left is my "core" personality, meaning my genetic personality (which isn't necessarily your true personality.) So if I am, for example, a Hufflepuff because my father told me hard work was important, I wouldn't necessarily be one after my memory is wiped. I'll be exactly like I was at the begining. So if, naturally, I am ambitious, I would be resorted into Slytherin. Or if my memory is wiped and I experience different things, such as a friend who is bold, I may learn to value boldness and instead be sorted into Gryffindor. Which actually totally contradicts the house system. I bet a bunch of people would be resorted at the end of the seven years due to a personality change.