it usually changes toward the middle of the drama and is usually due to fan reactions
Gotcha. I've only started delving deeper into kdramas, so I haven't noticed the patterns. But knowing that some of these shows can and do change writers midway, especially with their live-shoot system, I can see this happening more and more.

I've seen several dramas where a more famous woman has played secondary lead to a lesser known up-and-comer
Oh yea, true. But, if they were almost equal in acting experience, only one had slightly more star power than the other...I wonder how that would play out.
Thanks for your in-depth answers :)