Continuity mistake: Whenever young Riley is pulling the wagon around the house, Fear stops her at a white cord in an outlet. In the next shot, it is a black cord.
Continuity mistake: At the very beginning of the movie when Riley is born, the shot from behind shows the mother wearing the glasses. In the next shot the glasses disappear.
Continuity mistake: Bing Bong hurts his leg and cries candies. One with a grey wrapper vanishes in the next shot. (00:40:00)
Continuity mistake: On Riley's first day of school, she has a blue notebook on top of some other books on her desk. After her story about Minnesota, she sits down, where the notebook is now under another book.
Answer: Sadness can change the memories because of Riley's current situation: she has left the home she loved to move to a new city. This makes her happy memories more susceptible to being changed to sad memories. As she remembers things that used to make her happy they now make her sad because she hasn't accepted her new house as home yet. Presumably under the right circumstances the other emotions could do the same thing, just not all the time.