Historically, the norm by which the Popes regulated the development of the liturgies of the Church was tradition. In general, change was only introduced when the circumstances of the times objectively required it, and there were no complete, wholesale revisions of the liturgy in general. Changes were only ever minor, and they were genuine improvements on what came before. The substance of the liturgical rites was preserved, in accordance with tradition, but sometimes improved upon, in accordance with the necessities of the changing times.
The basic principle followed was that of St. Thomas Aquinas, according to which change in law ought only to be introduced when it was necessary to do so. Such a necessity might arise when the previous law in question is shown to have become in some way harmful or deficient by reason of newly arising circumstances. Thus, at all costs, the customary laws are to be retained, and changes only introduced when absolutely necessary.
This principle is restated by several of the Popes, including Pope Pius VI, who specifically quotes the authority of St. Thomas on this matter, stating that even the Popes strived to introduce change only when it could improve upon the existing discipline in general. Likewise, Pope Gregory XVI states that "that rule must be absolutely observed which states that, except for the most serious reasons and with the Apostolic See, no innovations are to be introduced into the holy rites of the liturgy."
It is important to note that, in regards to the liturgical tradition, it would be quite absurd to claim that the entirety and substance of the liturgy could ever fall subject to the necessity of change. Contingent circumstances could never affect the liturgy to such a vast degree that required the entirety of the tradition to be abandoned. Hence, whenever the necessity arose, it could only ever affect the liturgy in a minor way, thus requiring a change of correspondingly minor extent. The substance of the traditional rites would be preserved. Hence, Pope Pius XII, in speaking of the Ruthenian rite, indicates that the Holy See always safeguarded “everything essential to the rites and ceremonies of the Ruthenian Church,” while at the same time it “allowed or provisionally approved of some minor changes due to the circumstances of the particular times.” Pope Pius soon afterwards quotes the aforementioned words of Gregory XVI as an authority.
Furthermore, Pope Clement VIII writes that the changes introduced to the Roman missal under his authority were of such a nature that improved upon the substance of the traditional rites without abandoning them for something new. “These improvements, however, flowing as it were from the same sources and principles, seem rather to represent and complete the meaning of the rules and rubrics than to introduce anything new.” He writes that the pastoral care of the Papacy is to “preserve in everything the best and old norm.”
Pope Leo XIII, speaking of the Oriental rites, says that the Church only ever introduced changes into the liturgy which were in harmony with her venerable tradition, thus indicating that tradition ought to be preserved, even by the Popes, in the regulation of liturgical development.
It is evident, then, that the Popes have generally only regulated the development of the liturgy in such a way that sought to preserve the substance of the liturgical traditions, introducing only minor changes when necessity required.
It is possible to maintain that, throughout the Church’s history, there have been a few Popes who did not always perfectly fulfill their duties in this regard. For example, some traditionalists have criticized the reforms of Leo X and Clement VII, and the composition of the Quignonez Breviary, on the basis that these reforms were contrary to venerable tradition. Some traditionalists have also been critical of the reforms of Pius X and Pius XII and certain others, for the same reason. Whether such criticisms are valid or not, it is nonetheless evident that the principle remains always the same – as Pius XII himself enunciated – namely that changes ought only to be made which are minor and contribute to the authentic improvement of the liturgical tradition. Whether or not Pius XII himself succeeded in remaining faithful to this principle in practice, it is evident that he held it in high esteem, and therefore that it ought still to be held with such esteem. Tradition must be regarded as the norm in liturgical matters, and changes in the liturgy ought to be in harmony with the substance of the tradition. To overthrow nearly an entire tradition of liturgy could never be right.