To say that plots change while a story is being worked on is kind of like saying water is wet and the sky is blue.
Is it a bad thing? Not really because many reasons can contribute this phenomena rearing its ugly head. Perhaps the writer has grown bored with the subject material and wishes to take it in another direction. Perhaps the writer is stuck at a certain point (the first paragraph tends to be my personal Torquemada) and wants to try different things to get things moving again. Perhaps the writer is simply going through a bout of procrastination and is trying to compensate by moving words around.
The main problem with making changes while a story is in work-in-progress mode is that it delays finishing the story and when that happens, time tends to fly by faster than one might expect. Next thing you know, years have passed and one is left scratching their head in a most perplexed manner.
Could it be a good thing? Perhaps if the changes stick and a new story emerges from the old. I’m sure it happens but I wouldn’t put any wagers on it.
Having a solid and well-defined outline can help to keep one on track but in the end, remaining faithful to the spirit of the story is the most important thing to remember.
So, pour your favorite beverage, organize your workspace, and get to it. Stories don’t write themselves.