In Harry Potter, when a witch or a wizard is 11 they go to Hogwarts and their first major experience is their own Sorting. In front of the Great Hall, each new student goes to the stool, sits down, and puts on ye olde Sorting Hat. After anywhere from just a moment to over five minutes, the hat yells out one of Hufflepuff, Gryffindor, Ravenclaw, or Slytherin. And from then on that student belongs to that house.
But is that really it? Does the Sorting Hat's decision represent the house someone would belong to their whole life? Of course, you can't literally switch from one house to another while you're at school, but in spirit (which is what the Hat is meant to consider), can your house change?
Suppose some witch is sorted into Hufflepuff. More than that, suppose she is a true Hufflepuff, the most loyal, hard-working, and honest kind of person you'd find anywhere. Would it be possible, say fifteen years later, for her to go through some serious event in her life and come out not quite as loyal or hard-working, but so brave, chivalrous, and reckless that she'd suddenly be a true Gryffindor? Sure, she can't be re-Sorted, but if this were possible, then if you were to put the Hat on her it say she should be in Gryffindor.
So can this happen, or is it impossible for someone's character to change such that their house changes? Or, if this were possible, would the Sorting Hat have actually already known that fifteen years later the witch would become a Gryffindor, and so would have Sorted her into Gryffindor from the start?