You know, saying that the New Testament trumps the Old Testament is just another way of saying that Jesus is more authoritative than Moses, which is spelled out quite explicitly in the letter to the Hebrews. Consider these verses:
In the past God spoke to our forefathers through the prophets at many times and in various ways, but in these last days he has spoken to us by his Son, whom he appointed heir of all things, and through whom he made the universe. (Hebrews 1:1-2)
Jesus has been found worthy of greater honour than Moses, just as the builder of a house has greater honour than the house itself. (Hebrews 3:3)
Moses was faithful as a servant in all God’s house, testifying to what would be said in the future. But Christ is faithful as a son over God’s house (Hebrews 3:5-6)
So when it comes to questions about war, or leadership, or tithing or economics, who’s teaching takes priority?