What effect can changing the pacing of a story have?

That depends.

If changing the pace of a story: slow pace
The effect could be the reader getting bored of a slow story and/or the reader would not intrigued to a slow story.

If changing the pace to middle pace
The reader would likely be excited to see what occurs next in the story.

If changing the pace to fast: the reader could feel like the story is too rushed, and probably didn't enjoy the story enough. And probably wasn't excited- finishing the story as much.

Hope this helps.