When the Lord measures an individual, He does not take a tape measure around the person’s head to determine his mental capacity, nor his chest to determine his manliness, but He measures the heart as an indicator of the person’s capacity and potential to bless others.
Marvin J. Ashton, "The Measure of Our Hearts", General Conference, October 1988
I've often pondered the phrase "change of heart". I used to think of a change of heart as a one-time event where we change our heart from desiring bad to desiring good.
But that must be a daily occurrence. Every day we should have a change of heart. Every day we change our hearts to be closer to being like the heart of Christ.
We do this through daily repentance (which also has a negative connotation). But repentance means change. We literally pray and tell Heavenly Father we want to change. We tell Him what we did that day and how we're going to change for the better. It doesn't necessarily mean that we will change right away, but the desire to change is there. The heart is willing to.
That's all that Heavenly Father requires of us. To be willing to change. Overtime, that willingness will move us forward, little by little, as we actually change.