A change of heart
Definition 1 of 1
Pro Tip 1/3Emotional Shift
Understand it as an emotional or moral decision, not just a logical one. 
After seeing the puppy's sad eyes, she had a change of heart about giving it away.
Pro Tip 2/3Unexpected Decision
Often suggests a decision that is surprising to others or a reversal of a previous stance. 
Despite his strict policy, the teacher had a change of heart and accepted late assignments.
Pro Tip 3/3Personal Growth
May imply someone has grown or learned something new that influenced their perspective. 
She had a change of heart about her career after volunteering abroad.