The Pope can change church law but can he change God's law?
Well, even the ritual law of the Old Testament was established by God and therefore considered "God's Law". And we know that ritual law has ceased. So not everything is intrinsically contrary to Gods will and constitutes immutable divine law. So, for instance it's undoubtedly true that many good Catholics were vaporized by the nuclear bomb at Nagasaki. God is able to resurrect bodies regardles of how thoroughly they were destroyed. God could resurrect bodies out of stones, to paraphrase the New Testament. Our Lord also stated at one point, "Let the dead bury the dead." ... which suggests that this law fits more with the ritual law of the Old Testament.
While the respect due to the bodies of Catholics (former temples of the Holy Ghost) and the symbolic hope of and belief in a future resurrection both commend the practice, it does not appear to be considered immutable divine law by the Church. Certainly a just cause might permit cremation.