I know we do not like the word obligation. I do not like it either. But I still (even as a Byzantine) agree that there is sin present in willfully missing a Holy day or Sunday without an illness or other adequate reason (yes there can be a few of course). So though the term seem weird, because we should want to go. It still makes sense on a certain level, and I do not mind using that language here for now.
Now let me be up front. If I am wrong, I apologize. I would hate to be responsible for misleading anyone. But what I share hear is my current belief. I am not a canon lawyer.
Someone asked me to show them in canon law why I believe like I do. But with all due respect. I think that the burden of proof would be on you to show me in canon law that there can be exceptions to following the normal canon law for the rite that you technically still belong to, just because you practice at another rites parish. Without this being specifically mentioned, then I do not see how you can hold to it being that case.
Catholicism is a religion where we believe in a visible church with visible leadership. Obviously the Pope is the guy at the top, but for most of us, the person put in charge of each of us and our pastors would be our Bishop. Because our church is visual I believe that every person must be under the authority of a Bishops care. Someone that would be able to formally recognize this. This is why I think that one is expected to be faithful to one's Bishop's authority and practice even if discerning joining an Eastern Church. Because until it is formally recognized that you have made a change of rite, you must be technically under some visible persons authority and canon laws. I do not think it seems Catholic to me to just personally determine that you do not like that person so you will ignore their authority and follow another's. What is to stop you from changing back and forth every three months?
I spent a year or more in an Eastern Catholic parish before formally changing rites. I know of the importance of fully trying on that outfit before deciding to purchase it. There is a lot to it and it does take time to discern if it is right for you. But I think that one can do that simply by keeping the extra disciplines and feasts that it has that do not interfere with latin rite practices. Such as the Dormition or St. Peter and Paul fast or lenten practices etc. But there is at least one practice that a Latin Catholic simply can not participate in until they are fully incorporated into the Church. Say that you have small children. If you are Byzantine they are free to take communion from infancy. But say you are discerning if you want to become Byzantine, and therefore you are emotionally and spiritually trying to follow Byzantine customs and calendar etc. You would think that part of discerning if it was right for you would be to bring your children for Communion, since this is what you would have to believe and want to participate in in order to know if this Church was right for you. But unless you are Byzantine formally, you can not bring your children for Holy Communion in a Byzantine Church if they have not made first Communion in the rite that they belong to.
Lastly, say that a Byzantine member was discerning changing rites to the Latin Rite. Would you recommend that they then ignore the fasts of the Church that they still formally belong to in order to discern what being a Latin Rite Catholic means. After all, Catholics seldom if ever fast on wednesdays or abstain from meat and dairy or fast before the dfeast of St Peter and Paul etc.
Again, I am not saying that I know the answer but I am presenting my case for what I think the answer is. If I am wrong, I would love to be able to be shown that I am wrong.