The commonly posted advice on this forum as the first step to changing rites/sui juris churches is to live according to the spirituality the new church for a period of time. When doing this, would the person still be obliged to observe the canons (e.g., fasting) of their old church? Sorry if this is a dumb question.
It’s certainly not a dumb question, but surely a potentially complex one. There are many potential scenarios. Talking with one’s spiritual father is always important.
The simplest change can occur due to marriage, where no permissions are needed to make the change.
In the Latin Church Catholic either a man or woman can change
CIC
Can. 112 §1, 2/ a spouse who, at the time of or during marriage, has declared that he or she is transferring to the ritual Church sui iuris of the other spouse; when the marriage has ended, however, the person can freely return to the Latin Church;
An Eastern Catholic woman may transfer to the Latin Church of her husband
CCEO
Canon 33 A wife is at liberty to transfer to the Church of the husband at the celebration of or during the marriage; when the marriage has ended, she can freely return to the original Church sui iuris.
Apart from marriage, it is basically unheard of for an adult who is ascribed to an Eastern Catholic Church to be given permission to leave the ECC and become ascribed in the Latin Church.
Should that person still follow the calendar and praxis of his or her EC Church althought living as a Latin Catholic? Than depends. We have on CAF an adult who was Orthodox by virtue of their parents having entered the Orthodox Church. As an adult this person chose to be received into the Catholic Church. Because s/he was Orthodox coming into full communion she automatically was a member of the EC Church
sui juris closest to that Orthodox Church.This person’s family never actively practiced their Orthodox faith. They didn’t follow the calendar, the fasting, etc of their Orthodox Church. The person’s own liturgical life has been in the Latin Church, which s/he loves. Clergy s/he has consulted, and individuals here have said s/he should live as a Latin Catholic. Period.
For Latin Catholics wishing to change to an ECC or OCC, apart from the previously mentioned change due to marriage, they do need to be actively living as a faithful Catholic of the EC Church of which they are now a parishioner. Since our EC fasting is typically stricter than the Latin Church, both the Eucharistic fast, and the fasting periods before a Feast, and since our calendar typically contains more services than the Latin Church (Vespers as integral to Divine Liturgy on Feasts and Sundays, Pre-sanctified Liturgies, Paraklesis during Dormition Fast, major feasts still celebrated, like Dormition, which are no longer an “obligation” in many Latin diocese, etc.) it would be hard to picture living as part of the ECC, as a Latin, and to be failing to not do more than what the canons of the Latin Church expect.
Can you give an example of a canon of the Latin Church which one would be violating if living their life as if they were an EC? Maybe then we could better answer what you’re asking.
What about holy days of obligation?
Which Holy Days of Obligation are you thinking of? These vary in the Latin Church according to the country and according to the Diocese. Many countries have only two such Days, as does Hawaii. This doesn’t include Pascha/Easter which of course is the Feast of Feasts and all will joyfully wish to celebrate.