Dad's Anger: Uh-oh, she's looking at us. [Turns off TV] What did she say?
Dad's Fear: Uh, sorry, sir, no one was listening.
Dad's Anger: Is it garbage night? Uh, we left the toilet seat up! What? What is it, woman, what?!
Sadness: You could get lost in there!
Joy: Think positive!
Sadness: Okay, I'm positive you will get lost in there.
Joy: All these facts and opinions look the same. I can't tell them apart.
Bing Bong: Happens to me all the time. Don't worry about it.
Bing Bong: Take her to the moon for me. Okay?
Bing Bong: I cry candy. Try the caramel, its delicious.
Fear: Maybe it was a bear?
Disgust: There are no bears in San Francisco.
Anger: I saw a really hairy guy, he looked like a bear.
Sadness: Crying helps me slow down and obsess over the weight of life's problems.
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.