There’s a time in every manuscript for little changes that have big impact.
It comes in the third or fourth draft, when you’ve been working on a book for a while and the characters are solidly in your head.
Usually, these little changes occur to you when you’re finished writing or editing for the day.
They often appear when you’re in a reflective, what-if frame of mind. This is different from the what-if mood of the start of a book when everything seems possible. Everything is NOT possible now, only what works within the scope of the story.
These little changes are fun, but they should come with a warning sticker: “Check to make sure changes fit with, and resonate throughout, the book.”