I don't know why are audience unable to accept a changed Shagun, coz atleast for me this change in Shagun is very natural.
People does change even in real life, the biggest example is Ashoka, the Samrat who never - ever did think twice before beheading a person to get power but after seeing the bloodshed in the famous Kalinga war, he had a change in heart and accepted Buddhism and became a preacher.
Similarly, Shagun had a transformation in her heart after the Earthquake. If we really look at Shagun's life journey, it had been terrible. The lady was not permitted to enter the house by her BIL and she was taken home by Ishita, she was dumped by her chosen partner on the day of the marriage and then later thrown out of the house like a piece of garbage ( we were at that tine shocked to see that Shagun did not change even after such a huge thing).
Shagun was taken in by her ex- family but was then sent to her saut's mother's place and was at the end was insulted and thrown out from there.
Her kids got to know her real face and then both decided to throw her out of their life. After facing all these, even then if a person would not transform themselves then that is shocking. Normally what Shagun is doing is the path which usually all people like Shagun follows, which is get themselves associated to the society work to do penance.
So for me Shagun's 360 turn in her personality is totally normal. Her sudden love for her kids, yearning to see them, all are a very natural progression in her character and nothing out of it.
PS : The above are just my opinion regarding Shagun's changed behaviour, I would request u also to post it's but BASHERS not allowed.