He relented: If the people of Nineveh had not repented, God would have destroyed them (Jonah 3:4). But God was ready to show mercy when they repented (see Jonah 4:2, 11). In God's view, this change did not reverse his original plan, because he always considered the possibility of mercy. This change does not affect God's ability to know the future.
Historically, the church has believed that God fully knows the future (see Psalm 139:4; Isaiah 46:10; Daniel 2:28–29; Matthew 24:36). Nothing in this account contradicts that belief.