I'm hoping it isn't a hand of god thing this time, though it is a sequel..... I've never understood how changing the past doesn't change the present, so I wonder if they'll get into that?
So, by and large, changing the past
DOES change the present. At one point Sam chooses to
not save Al's first marriage, because that change would have unpredictable impact on the Quantum Leap Project, and thus everything Sam had done so far in the series.
Often, the changes in history are largely personal - much of the original Quantum Leap is personal stories about the well-being of individuals. Like, does a particular nun get to build a chapel that has become her life's dream, or not?
When the results would be visible to the public, they are generally changes into the world the audience already knows (and often the one Sam knows).
For example, in an episode I watched just last night, Sam thinks he's there to get the guy he's jumped into to marry a woman. Then, it looks like he's there to make sure she marries this other guy. This is a typical QL story. But then, he leaps after he gives a young man named Buddy that's helping him out that maybe the song he's noodling with should be about a girl named "Peggy Sue". The implication being that Sam put Buddy Holly on the right track.
Never mind how a-historical that is. The episode is explicitly set in 1956. By 1956, Buddy Holly wasn't noodling on a guitar and running odd jobs for a veterinarian. He was opening for Elvis and Bill Halley and the Comets. And never mind that this means Sam is setting Buddy up for death a mere three years later in 1959...
The tagline is there: Putting right what once went wrong. What they don't tell us is how it went wrong. At least, until the Evil Leapers show up...