St. Thomas discusses in the Summa the changeability of human laws, pointing out that any change in custom is naturally disturbing to men. This principle is recognized universally by scholars and theologians, Catholic or not. Custom and habit are not meant to be broken, and man is a naturally habitual creature. Novelty is by its nature disturbing to men. For this reason, St. Thomas and St. Augustine, and other Fathers of the Church, point out that any change in custom, even if it is beneficial in some way, will be psychologically disturbing to men - and likely spiritually disturbing as well, in the context of the Catholic tradition. For this reason, custom ought not to be changed, in general.
But there seems to be more to what St. Thomas says of this subject. In the context of law, a custom isn't simply a habitual practice of the people, but something which is formally and officially sanctioned or imposed by a higher authority. This adds a further dimension to the problem. In such a context, where custom is protected by law and founded on authority, when such a custom is changed, the consequent disturbance is even greater. Not only is there the natural disturbance which results from any novelty whatever, but there is also the disturbance which results from being subject to an authority which fluctuates and contradicts itself for no apparently reason of necessity. An authority which is inconsistent in this way appears to be incompetent, to be ignorant of what it is seeking to achieve, or how to achieve it. Hence, St. Thomas says that "when a law is changed, the binding power of the law is diminished, in so far as custom is abolished." This is immensely important: authority and law cannot be separated from custom and habit if they are to retain the binding character which is proper to them. Custom is not simply advantageous to the people, being the habitual practice, but advantageous also to those in authority over the people.
The consequences of this principle are likewise immensely important. When authority fluctuates and is inconsistent, one can only imagine how the people will react. It will be chaos. And indeed, in the current crisis in the Church today, this is exactly what one may see. Take, for an obvious example, the problem of liturgical abuse: the reason that so many in the Church do not follow the rules which the Church has laid down for the celebration of the liturgy is likely because the Church herself seems not to know what it is she wants. This is the reason for the widespread lack of discipline and even of sound doctrine among the priests and bishops of the Church. When authority fluctuates, the people slowly and subconsciously begin to succumb to the Protestant rule of private judgment, since there is no norm which presents itself as consistent or reliable enough to be followed with benefit.
This is one reason why tradition is so important: it provides consistency. Even if tradition itself is founded on the authority of the Church, and so, technically speaking, the Church does possess the legal right to introduce changes, nonetheless the Church must still exercise prudence. And reason itself reveals that the most prudent authority will also be, to the greatest extent possible, the most consistent. This means that, as best as possible, continuity with the past will be preserved. The logical consequence of this is the preservation of tradition, as the rule and norm of ecclesiastical authority. Therein lies the solution to virtually all of the Church's problems today.