Immutability is the concept of the impossibility of change. This results from Perfection. Only God possesses this quality. No change in His Being or in His Laws because these are not necessary. The Laws are perfect. God is Perfect, so why change? The concept of change does not apply to God at all. Change implies improvement thereby imperfection. Perfection on the other hand implies immutability.
As mentioned earlier, see “Death of Jesus”, God does not need to change even one particle of His Laws because these are perfect. He had, right from the beginning taken all happenings into consideration so that nothing is too much or superfluous for His Laws to deal with. That degree of comprehensiveness is what makes His Laws perfect and as such no need for any changes.
As human beings we always have to keep changing our earthly laws to accommodate every new happening because it is quite impossible as intellectuals to foresee everything and make laws for these happenings beforehand. In any case the World of Matter is always subject to a change in its form but the Laws of God which penetrate and stand above this are immutable.