Hinted at in the Score Contradictions section directly above, was the recommendation for you to be extra cautious whenever you decide to make a manual override to one of your Learner's final overall Course Scores.
This may sound a bit drastic, but understand that for a Learner that has finished their Course, the LMS will no longer calculate an overall Score automatically. Translation: Any overrides you make here after your Learner moves on, cannot be undone.
If you did ever want to return to a Learner's original overall Course Score, you would have no choice but to recreate it yourself.
Now once you start remembering that a large, graded Course can easily have a very complex scoring formula attached to it (with all the possible scored Lessons and their individual scoring weights used to obtain a Learner's final mark), this warning becomes pretty apparent.
If that doesn't convince you, just consider the possibility for frustration and error if you have to ever go back to a Learner's Lesson Activity (while also referring to how the authored Course calculates its grades!) and manually recalculate their original overall Course Score, that you chose to replace.
So now that you know, definitely make sure...that you're sure - before you change it.