35 From this brief historical summary it is easily seen with what care the Apostolic See has watched over the integral preservation of the Ruthenian rite, both as regards the community as a whole and as regards individuals. However, no one will be surprised if it has permitted or temporarily approved some minor changes on account of the special circumstances of the times, provided always that the chief and essential rites remained whole and entire. Thus, for example, it has permitted no changes to be made in the rites of the liturgy, save the few decreed by the Ruthenian bishops themselves in the council of Zamosc.
Source: Orientales Omnes Ecclesias (Vatican.va)