The Second Vatican Council Document Orientalium Ecclesiarum, The Decree on the Catholic Churches of the East, says:

“All members of the Eastern Rite should know and be convinced that they can and should always preserve their legitimate liturgical rite and their established way of life…” (no. 6)

and…

If any separated Christian should, under the guidance of the grace of the Holy Spirit, join himself to the unity of Catholics, no more should be required of him than what a bare professional the Catholic Faith demands. (no. 25)