In baseball, the change-up is a pitch a bit slower than the fastball. The batter, expecting the fastball, swings early and WHOOSH. Strike-three. You’re out.
In writing, however, I use the term “change up” to mean a significant shift in mood. TV tropes calls it “Mood Whiplash” and “Mood Dissonance”.
Joss Whedon is a master of it. (See Dr. Horrible’s Sing Along Blog as a good example.)
The Change Up is an important tool in fiction. Let me give you an example, not from fiction writing itself but one that illustrates the point.
When I first got an MP3 player, I loaded it up with my absolute favorite songs. Now, at the time, my tastes ran to rather sentimental love songs and ballads. Soon I had a playlist consisting of all my very favorite songs.
Only one problem. The playlist was boring. There wasn’t anything wrong (for my tastes at the time) with any of the songs on It--remember, they were all among my top favorites--but the same basic themes and styles over and over again became monotonous. The fix, in that case was simple, to go further down my list of favorites and find some music with different styles and different themes to add to the mix. The result was a far more interesting, and less boring, play list.
I find the same thing happening in fiction. I like gritty, realistic military SF. Such fiction often tends to be grim. But the fiction I’ll come back to, the series I’ll stick with, and the writers I will continue to follow, are those who add a bit of lightness to the mix--humor, perhaps, or maybe some romance.
So don’t be afraid to mix it up a bit. Add some levity to your dark and gritty story. Get serious occasionally in your lighthearted comedy. You may find your story all the better for it.