Yes, the emotional impact of the story will change. It will change to a melancholic mood.
In literature, this is called mood.
The mood of a text refers to the psychological and emotional triggers caused by the content of a story or text.
The mood of a Christmas story is always celebratory.
If however, during that Christmas story, a child dies unexpectedly, and then there is the subject of mourning and burial and the aftermath of such even on the parents of the child, the mood of that story cannot remain ecstatic.
The writer would have introduced a mood of tragedy. Tragedy and ecstasy don't feel the same.
Click the link below to learn more about literary moods: