1)
In what sense did Hashem switch for him a different heart?
1.
Rashi: It was a Ru'ach of Gevurah of kingship.
2.
Radak: Refer to 10:6:4:1.
3.
Midrash Mishlei 21:1: "Palgei Mayim Lev Melech b'Yad Hashem Al Kol Asher Yachpotz Yatenu" - when a person becomes king, his heart is in Hashem's hand. This is like water in a Kli - one may tilt it to any side he wants. Malbim - a person's heart conducts his bodily powers. The king's heart conducts the hearts of the people under him. Hashem veers his heart, and the hearts of the people under him, as He wants. Now Sha'ul's heart applies to Klal Yisrael.
