God does not change Num 23:19, and neither does His character. The idea that loving our enemies is somehow a New Covenant principle that was absent in the Old Covenant is simply incorrect.
The Jews were also commanded to love their enemies. Consider Exodus 23:4-5, which instructs them to help even an enemy whose animal had gone astray or fallen under its burden. Leviticus 19:18 commands them to love their neighbour, and Proverbs 25:21 explicitly instructs them to feed their enemy if he is hungry and give him water if he is thirsty. These principles existed long before the New Covenant.
Furthermore, leaving vengeance to the Lord does not mean that justice is abolished. Romans 12:19-21 teaches us not to take personal vengeance, but Romans 13:1-4 makes it equally clear that governing authorities are appointed by God to administer justice and punish wrongdoing.
Self-defence is also a separate issue entirely. Loving your enemy does not mean that you must passively submit to violence or that defending innocent people is somehow contrary to Christian charity.
We should be careful not to confuse personal hatred and vengeance with legitimate justice, nor should we pretend that the Old Testament taught a different moral character of God. God has always expected His people to act justly, show mercy and love others, including their enemies.
