30 The tireless and solicitous care of the Roman pontiffs for the preservation and protection of the Ruthenian rites is best seen from the course of the long drawn out question of the change of rite. Although, for special reasons which were utterly alien to their will, they could not through a long period impose on the laity an absolute prohibition of this change, none the less their repeated efforts to establish that prohibition, and their exhortations to the Latin bishops and priests, clearly show how much our predecessors had this matter at heart. In the actual decree which in 1595 happily established the union of the Ruthenians with the Apostolic See there is no clear and open prohibition of going over from the Easter to the Latin rite. Nonetheless, what the mind of the Apostolic See then already was is shown by the letter sent in 1608 by the general of the Society of Jesus to his subjects in Poland. He tells them that those who had never belonged to the Latin rite could not embrace it after the reconciliation, "because it was the precept of the Church, and was specially laid down in the documents of the union under Clement VIII that everyone should remain in the rite of his own Church."
Source: Orientales Omnes Ecclesias (Vatican.va)