Here's a nice clip from The Town that's a great example of changing emotions
The girl starts out very flirtatious - playing with her earring, smiling, pressing her lips together. She's never met this guy and she's putting on the usual "I'm interested" act
Then he starts talking about the 20 dollar bill. She starts a little confused, she's never heard this one before, but then she genuinely starts to get more interested. The smile she starts to use now is more genuine than the one she had before. This is a subtle change of emotion - she's went from I'm pretending to be interesting to I'm more genuinely interested.
Finally he mentions the oxy (drugs) and she realizes this person is not who she thought he was, she instantly starts to avoid eye contact, slouches down in her seat, you can feel her getting a little nervous/worried.
Before you have a change of emotion, its helpful to build up a different emotion first, basically its anticipation but with feelings. If you were to animate a character jumping, you would first anticipate them down. The same concept can apply to emotional changes, if you want to have a more drastic change, have the character go from two opposite emotions. (anticipations are usually in the opposite direction of the main movement). So if a character is going to be frightened, have them start more happy/calm. If they are going to laugh from a joke, maybe they start upset/mad. Creating contrast in your emotion change will make for a more interesting animation.
Acting Reference #55 from Kyle Kenworthy on Vimeo.