How does Mrs Weatherall character evolve over the course of the story
Considering that the story is narrated as a stream of consciousness, one might want to start stating that she is narrating from one same setting, but jumping from memory to memory.
It would not be 100% accurate to state that Granny went through a complete change. If we part from her biggest life changing event, the day she was jilted at the altar, we notice that her entire life revolved around that incident: One which she quotes "she has prayed for 60 years to forget about". It is obvious that the incident sank her into a deep state of despair which, in turn, ruled every decision she made thereafter.
So, she went from a hopeful bride who got jilted to a woman who finds a new husband and devotes herself entirely to make their family happy. Yet, we see cracks in that solid surface.
She obviously did all that she did in life to avoid thinking about the jilting. She would attempt an exaggerated normality which may have obsessed her to the point of neglecting other things. At all times, every decision she made was away to demonstrate to herself that she had gotten over it. She shows regrets. There was a point in her life when she got sick and she remembers listing down (like now) whether she had done all she had to do with her children and her late husband. There is obviously a sign of unfinished businesses.
In the end, she realized how every single memory of her life has been opaqued by that fateful day when the cake was thrown away wasted, when she had worn the white dress and veil, and when she waited endlessly for her former fiance to show up at the church. As she is still trying to shake that off, despite trying to convince herself that the incident did not affect her, she deflected her thoughts once she saw that the moment of death came near.
At this point, she began to wait, again, for a "sign" to show up. This time, it was not a sign of whether her fiance would show up, but a sign of whether God would show her a sign of the afterlife. It did not come. Just like her fiance did not show up. This is when she went in despair and felt jilted for the second time. Only worse.