"Non nova sed noviter"

On the topic of "reform" in Church teaching, I think the following is especially helpful, emphasizing, as it does, an "inviolable law":

Not without serious reason, Venerable Brothers, have We wished to recall these things in your presence. For unfortunately it has happened that certain teachers care little for conformity with the living Teaching Authority of the Church, pay little heed to her commonly received doctrine clearly proposed in various ways; and at the same time they follow their own bent too much, and regard too highly the intellectual temper of more recent writers, and the standards of other branches of learning, which they declare and hold to be the only ones which conform to sound ideas and standards of scholarship. Of course the Church is very keen for and fosters the study of human branches of learning and their progress; she honors with special favor and regard learned men who spend their lives in the cultivation of learning. However matters of religion and morals, because they completely transcend truths of the senses and the plane of the material, pertain solely to the office and authority of the Church. In Our encyclical letter, Humani generis, We described the attitude of mind, the spirit, of those whom We have referred to above; We also recalled to mind that some of the aberrations from the truth which We repudiated in that Encyclical had their direct origin in a neglect of conformity with the living Teaching Authority of the Church. Time and again St. Pius X, in writings whose importance is known to all of you, urgently stressed the need for this union with the mind and teaching of the Church. His successor in the Supreme Pontificate, Benedict XV, did the same; in his first Encyclical, after solemnly repeating Pius' condemnation of Modernism, he thus describes the attitude of mind of followers of that doctrine: "He who is influenced by its principles disdainfully spurns whatever appears old, and eagerly pursues the new: in his manner of speaking of divine things, in performance of divine worship, in Catholic usages, even in private devotions" (AAS VI [1914], 578). And if there are any present-day teachers making every effort to produce and develop new ideas, but not to repeat "that which has been handed down," and if this is their whole aim, they should reflect calmly on those words which Benedict XV, in the Encyclical just referred to, proposes for their consideration: "We wish this maxim of our elders held in reverence: Nihil innovetur nisi quod traditum (Let nothing new be introduced but only what has been handed down); it must be held as an inviolable law in matters of faith, and should also control those points which allow of change, though in these latter for the most part the rule holds: non nova sed noviter (Not new things but in a new way)."

Venerable Pope Pius XII 

Allocution Si Diligis to Cardinals, Archbishops and Bishops following the canonization of Saint Pius X 

May 31, 1954