Sacrosanctum Concilium §23

²³ That sound tradition may be retained, and yet the way remain open to legitimate progress careful investigation is always to be made into each part of the liturgy which is to be revised. This investigation should be theological, historical, and pastoral. Also the general laws governing the structure and meaning of the liturgy must be studied in conjunction with the experience derived from recent liturgical reforms and from the indults conceded to various places. Finally, there must be no innovations unless the good of the Church genuinely and certainly requires them; and care must be taken that any new forms adopted should in some way grow organically from forms already existing. As far as possible, notable differences between the rites used in adjacent regions must be carefully avoided.